Summary: Initially, OP's configuration didn't work for unknown reasons, but now it seems to be functioning perfectly after some thorough testing.

Hypothesis

I speculate that the issue might have been caused by inadvertently using the wrong tsconfig. Upon reviewing the repository's tsconfig, everything appears correct. Could there have been interference from another configuration file during those runs? I noticed that there were no automated tests present.

Experimentation

Encountering a similar problem today, I conducted a quick test utilizing OP's model. I extracted compiler options from the official documentation.

decorators.ts

export function logProperty(target: any, key: string) {

    let val = this[key];

    const getter = () => {
        console.log(`Get: ${key} => ${val}`);
        return val;
    };

    const setter = (newVal) => {
        console.log(`Set: ${key} => ${newVal}`);
        val = newVal;
    };

    if (delete this[key]) {
        Object.defineProperty(target, key, {
            get: getter,
            set: setter,
            enumerable: true,
            configurable: true
        });
    }
}

main.ts

import { logProperty } from './decorators';

class Person {
    @logProperty
    firstName: string;

    @logProperty
    lastName: string;

    constructor(firstName: string, lastName: string) {
        this.firstName = firstName;
        this.lastName = lastName;
    }
}

const foo = new Person('Foo', 'Bar');

function logProperty2(target: any, key: string) {

    let val = this[key];

    const getter = () => {
        console.log(`Get: ${key} => ${val}`);
        return val;
    };

    const setter = (newVal) => {
        console.log(`Set: ${key} => ${newVal}`);
        val = newVal;
    };

    if (delete this[key]) {
        Object.defineProperty(target, key, {
            get: getter,
            set: setter,
            enumerable: true,
            configurable: true
        });
    }
}

class Person2 {
    @logProperty2
    firstName: string;

    @logProperty2
    lastName: string;

    constructor(firstName: string, lastName: string) {
        this.firstName = firstName;
        this.lastName = lastName;
    }
}

const foo2 = new Person2('Foo', 'Bar');

index.ts

Outcomes

To observe the complete build process, visit Travis.

Robust tsconfig

Based on the outcomes, it seems that this tsconfig is acceptable.

Concluding Remarks

• I may not have tested all potential compiler options thoroughly. If desired, I can update the repository with additional tests later on.
• The issue encountered by OP still perplexes me, as I haven't found a definitive solution.
• I also verified the TypeScript version used. While OP utilized 2.4.2, I employed 2.7.2. To rule out version-related issues, I downgraded my version as well.

The reason why 'this' is not defined is due to improper configuration of the property descriptor.

Instead of using:

Object.defineProperty(target, key, {
      get: getter,
      set: setter,
      enumerable: true,
      configurable: true
    });

You should consider doing something like this:

Object.defineProperty(target, key, {
      get() {
         // 'this' will be defined here
      },
      set(value: any) {
         // 'this' will be defined here
      },
      enumerable: true,
      configurable: true
    });

I encountered the same issue and adopting the solution provided above resolved it for me.

Here is a helpful tip:

Instead of using the following code snippet:

function logProperty(target: any, key: string) {

 ...
}

Consider using this alternative approach:

const logProperty = (target: any, key: string) => {
 ...
}

This change utilizes => which scopes 'this' just outside, allowing it to be accessed. I hope you find this useful!

I took inspiration from allen wang's code and modified it to create my own solution:

export const logProperty = (target: any, key: string) => {
    let val = this?[key]: '';

    const getter = () => {
        console.log(`Get: ${key} => ${val}`);
        return val;
    };

    const setter = (newVal) => {
        console.log(`Set: ${key} => ${newVal}`);
        val = newVal;
    };

    if (val) {
        Object.defineProperty(target, key, {
            get: getter,
            set: setter,
            enumerable: true,
            configurable: true
        });
    }
}

I encountered a similar issue, but with some strict conditions in place.

This was how my code looked:

namespace StringUtil {
  export function validate(...) {...} 
}

export function mv_string(minLen: number, maxLen: number, allowNull?: boolean, errCode?: string, errMsg?: string) {
    return function (target: any, propertyKey: string) {
        let cls = target.constructor as Function;
        createValidator(cls, propertyKey, StringUtil.validate, [minLen, maxLen, allowNull], errCode, errMsg);
    };
}

class MyCls {
    @mv_string
    myProp: string;
}

Previously, I had been using typescript 3.7.0+ with an older ts-node version, and everything worked smoothly whether running through ts-node or webpack && node xxx.

However, after upgrading to typescript 4.7.0+ with a newer ts-node 10.0.0+ version, the webpack & node run still went well, but ts-node | ts-node-dev started giving me a runtime error:

/Users/xxx/xxx/node_modules/reflect-metadata/Reflect.js:553
                var decorated = decorator(target, propertyKey, descriptor);
                                ^
TypeError: Cannot read properties of undefined (reading 'validate')
    at /Users/xxx/xxx/src/xxx/my-xxx-validator.ts:55:54
    at DecorateProperty (/Users/xxx/xxx/node_modules/reflect-metadata/Reflect.js:553:33)
    at Reflect.decorate (/Users/xxx/xxx/node_modules/reflect-metadata/Reflect.js:123:24)
    at __decorate (/Users/xxx/xxx/src/my-xxx-tscode.ts:4:92)
    at Object.<anonymous> (/Users/xxx/xxx/src/my-xxx-tscode.ts:6:5)
    at Module._compile (node:internal/modules/cjs/loader:1105:14)
    at Module.m._compile (/Users/xxx/xxx/node_modules/ts-node/src/index.ts:839:23)
    at Module._extensions..js (node:internal/modules/cjs/loader:1159:10)
    at Object.require.extensions.<computed> [as .ts] (/Users/xxx/xxx/node_modules/ts-node/src/index.ts:842:12)
    at Module.load (node:internal/modules/cjs/loader:981:32)

The error primarily pointed out that StringUtil was undefined when using StringUtil.validate.

I suspect the issue could be related to what @CJ Harries mentioned earlier, although I'm not entirely certain. It seems odd that the namespace would be compiled as an object while working with decorators via ts-node. Perhaps the upgrade introduced some breaking changes in this regard?