I manage four (4) email accounts with Thunderbird using unified folders. They are (3) POP and one (1) IMAP. Some, but not all, of the trash subfolders have recently vanished. The Trash main header displays all trashed mail but subfolders do not reappear until, only sometimes, the trash is emptied. At that, reappearance is temporary. Only one POP subfolder is displayed with consistency and has not vanished to my recollection. I noticed this behavior co-incident with spam arriving in the single IMAP folder with a frequency much greater than in the past.
Just noted that under "View" menu, selecting "All" instead of "Unified" restores all Trash folders and their contents. Re-selecting "Unified" keeps subfolders visible for a time, then eventually those revert to the vanished condition.