Import MMS pictures from old device

Hello Everybody

I know backing up MMS is an old topic (Jolla please fix it, or go forward and implement RCS), and there are many old questions with workaround, but I am still a bit stuck here:
Basically, the trick to transfer MMS to your new device seems to be to just copy over ~/.local/share/commhistory/data/ from the old to the new device after importing the messages from backup.
As far as I remember, this is what I did when switching from a Jolla C modified Aquafish to an Xperia X a few Months ago.
Unfortunately, I recently noticed that old MMS content is missing, so I copied the data again using rsync. Files were synced, but somethings seemed not right. I restarted the phone, and after the restart, all copied files were gone again. I tried again, with the same result.
What is deleting my MMS messages? How can I successfully transfer my MMS content to the new phone?