Hello
the method I have used is far from perfect.
I did query the database to get all files with invalid dates.
mysql <database name>
select path from oc_filecache where mtime <= 0;
from this I generated a script to put valid dates (but not from backup due to me wanting to be quick)
touch -c <filename reported by the query>
the hard thing is that path returned by the first query is missing the path to storage and I do not have an easy way to solve that
at the end I just trigger a scan of the files to update again the database
sudo -u www-data php <path to nextcloud>/occ files:scan