Kamikaze
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It depends on whether the hero was hired or given initially. You hired a hero; I selected mine when loading the map.
Try skipping a turn with your heroes - MP will be as I wrote. Or just place them on the map - the effect is the same.
And I didn't hire them in the tavern; I placed them in the town using the editor!!!
And how did you check the MP reserve?
Added after 3 minutes
I figured it out... :) Only the .xlsx file extension - mine doesn't recognize it. Excel must be in .xls format.