In the settings window, it simply does not allow you to select other textures as it was before and as it is written in the user manual.
The custom textures feature required Render To Texture using a custom module we wrote in DirectX 11, which also required other modules (DirectWrite, absent in DX12), and since P3D V5 works only in DX12, the feature is not available there but, of course, you have more than 350 handling operators to choose from. If there is one you are missing, just ask, it's fairly easy for us to add it.
This was explained at Page 70 of the GSX Manual for P3D, the paragraph "
Multiple Liveries" in the chapter which discuss Airport customization in P3D V5.