You are both correct; the problem is that the QAI version (MMH55_64.exe) is a bit older than the original ToE release (MMH55_Utility_64.exe), and at that time, wide-screen support was not included.
Thank you, dredknight! Am I right to understand that there is no way to fix this bug?
Added after 3 hours and 32 minutes
Another question, please: are there hotkeys in Heroes 5.5 for quickly splitting units? For example, for such an operation: 98 skeletons into 97 and 1 skeleton (like Ctrl + left click in Heroes 3 HotA). Or splitting a unit in half with some hotkey + left click?