What do you think it could be?

I would suggest that Alex_R play against this opponent again... and ask 1-2 other players to also play against this person. Make daily saves - and after such a "test," check everything again. And only then announce names and accusations ;)

There are a couple of nuances here... It's not a given that the person will agree to play - that's the first thing... And the second thing is that there are rumors in the Heroes community about a certain slow player, playing against whom turns into an endless wait. What this player is doing during this time is unknown. Maybe he's thinking about the battles... So, if the mysterious opponent Alex_R plays in the same style, it will be torture.