I found their explanation here:
https://github.com/kopia/kopia/issues/1764
https://github.com/kopia/kopia/issues/544
Still not solved after many years :(
Now I use Borg + Restic and I am happy
+ GUI for Restic https://github.com/garethgeorge/backrest
+ GUI for Borg https://github.com/borgbase/vorta