HoMM3 HD is a mod (add-on) for the game Heroes of Might and Magic 3.

And somehow it resembles self-isolation, as a player from any other version cannot connect to 3.25. Let them connect. Or is this a scheme to tell the player that there is 3.25? It's not comfortable.

What kind of fairy tales are these? Everything connects for me. I play in SOD through 3.25. And everything is fine with other versions of the mod. Maybe you have ND+ mode enabled? Then, of course, it won't connect to SOD.

regarding the case with a 30-level hero in a dungeon with weak guards - I can't imagine how HD could be involved in this.

I suspect that the reason for this is the fix for starting heroes who are not of the first level.