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

And it's definitely not a given, as everything was fine in the merchant window before.

This is a bug of Buka's Complete Collection. HD has nothing to do with this. (Run the game without HD and see for yourself)
In SoD from Buka, or in the English Complete, there is no such bug. Apparently, one of these was what it was "before".
I'll make a fix for this bug in SuperPack_Rus 1.4