OK, I found a solution.
You need to have Joplin installed onto the Android device. So if it is uninstalled, install it again.
Then, go to Manage apps (or Apps settings or similar) in your Android Settings. Find Joplin. Go to Storage and choose Clear Data.
All the internal data are now cleared and you can resync your Joplin on Android with your Joplin on PC (make sure to choose read only before you start on your sync client eg.: Syncthing on PC, so that the PC sync folder is not overwritten)
It worked for me.