I found after much investigation, that this happens because when Reality Converter converts models to usdz it uses the same names for textures. Then, when multiple models are merged into one USDZ there are name conflicts. As a workaround you can export all the images from each converted usdz file, rename the textures to something unique like '_base_***' and then drag them over the material texture slots in Reality Converter, then re-expoert the usdz, files then replace them in the reality composer project. (lol)