If I save files with CTRL-S, the Files pane is refreshed and the complete tree is collapsed. If files are auto-saved before a build (F7), the tree remains as it is. The latter shows that it is possible to have the tree remained open, which is highly preferable behaviour whenever a large document is split into many files that are sorted into directories and subdirectories. In this case, it is really tedious having to always reopen each subfolder after a save. The save-on-build workaround is not always applicable.
There has been an error report on this issue (ID: 3484888) in which it is claimed that this is a feature rather than a bug. I would kindly ask for an option to disable this feature in the TC preferences menu.