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The problem here is this: due to a developer error in the game, on large maps, the save file size exceeds (often) 30 MB (I might be wrong, this information is not mine), and the developers did not anticipate this, meaning the game stops loading such large save files. This is the cause of all the troubles.
How to fix it: most likely, there is no way. Maybe there is, but I don't know.
Advice: on maps larger than "large," save more often. Also, this is still a scenario, so anything can happen.
Well, I saved more often, but only on one save file))) Actually, I'm not an experienced player (although I've been playing the Heroes series since the first installment), so I didn't anticipate such nuances. The save file weighs 9 MB, by the way. Maybe you'll risk loading it on your computer?