Hi. It looks like there is a bug in the CubeTexture when using the files array to define the 6 sides of the cubemap. If you create multiple such cube textures you always get the first one created back. I made a small repro which can be found here: Babylon.js Playground
If you use the url parameter though everything works as expected. Please let me know if you need more info.
Oh, that’s not a bug.
This is because babylon caches and reuses textures bases on the texture name. This is a silly approach to naming texture, but if you give random names, you will see the texture changes when needed - CubeTexture files issue | Babylon.js Playground (babylonjs-playground.com)
Of course, a better approach would be to call it based on the index.
While waiting for an answer I already looked into your code and saw
this._texture = this._getFromCache(this.url, this._noMipmap, undefined, undefined, this._useSRGBBuffer, this.isCube);
which is called from CubeTexture::_loadTexture(). After checking _getFromCache() it was quite clear. 
Thx for the quick response though!