Thank you for your answer. The problem has been “solved”.
I noticed that all synced folders of the re-installed phone (which have been empty after the fresh setup) where empty on all synced devices! Obviously Syncthing cleared these folders everywhere because the imported database contained file entries in these folders but the folders on the re-installed phone where empty, of course. So the database must not be imported if one wants to restore the Syncthing settings on a re-installed phone.
I deleted the database, restored the lost files from an external backup and the problem in the OP was gone.
I created another issue with a feature request to provide a feature to restore Syncthing on a re-installed device. Feature to restore Syncthing on a re-installed device