This reeks of a bug in Eeschema. I’ve seen a few other threads recently, but in those threads fields were duplictated and appeared twice in the schematic.
It may be that the bug is triggered by a non-english GUI settings.
Can you try to:
Set KiCad’s GUI to English.
Open a copy of the old project again in a newer KiCad.
Look at the results.
Another way to try to resolve this is with a text file comparison program, such as for example meldmerge
That would not surprise me. I just had a bug happen in Eeschema, lost all (wow, all 3 of them) symbols after using the library editor… and no, I did not change 2 of the symbols that somehow got lost. Unable to duplicate the bug.