You cannot downgrade apps reliably on Android (unless the device is rooted, which allows you to fully back up and restore app data). With the Syncthing app’s built-in export/import functionality, you basically back the config up but not the database.
This means that if you were in the middle of synchronisation, e.g. you modified and deleted some files locally, and those changes haven’t been synced to other devices yet, then after you restore the config on a new installation, Syncthing will treat the modified files as conflicts, which you’ll need to resolve manually, and the deleted files will be re-downloaded from other devices.
Can you describe what the issue is specifically? There should be zero difference between these two versions of the app. Also, v1.27.10 is already available on F-Droid, so you may want to try upgrading to the current version and see what happens first.