I set up a project with a song and a part with a stack. I set the input of the stack to be a mono input that I have labelled as “gtr”. I can save the project, close down the app and then reload it and all is fine. If I create a second setlist within the same project and switch between this and the original setlist, then the stack will have reverted to the default stereo input.
Update: I have tried to recreate the issue with a much simpler test project but have been unable to. It must therefore be down to some corruption that occurred as I was building my actual project. I appreciate this will make it harder to replicate.