atyates27 I must be missing something, the STL format does not support layers.
Did you flatten the layer stack by any chance?
I can internally import STL files although this import isn't public yet, please get in contact through the support form on the website and I can get you the STL file in a forger scene.
Please note that STL files are triangulated so it's certainly not the best format for sculpting (which is why importing those files isn't an option) but it has been requested and I may add it soon 🙂