There is no way around that. Either state matches mappings= , or it doesn’t match.
A dirty way is to display state alongside buttons.
Switch item=ACRooms1 label="Fan Speed [%s]" ...
That could enhanced with a MAP transform, to show OFF/MED/HI etc. instead of numerics.
Does anything bad happen if you do send say, 33 instead of 26?
You could have your mappings= send 33, and have a rule listen for command 33 and shortly afterward send the “correct” command 26.
If you need to avoid the “bad” commands altogether, then you could use a proxy Item on your sitemap with any values you like, and a simple rule to pass corrected commands to the real Item.