I posted this on Skyrim Nexus already, adding it here for increased awareness. It's quite annoying, so any help would be deeply appreciated.

For some reason, any power attacks I do with my left hand, when unarmed, on two of my characters don't cost any stamina. It's specifically the left-handed ones, and it seems to be applying retroactively, i.e. all saves with those characters are affected. For some reason, power attacks with any weapons are unaffected, but my current build is Unarmed so that doesn't really help.

Notably, a test character I made doesn't seem to have the glitch, but nothing seems to be fixing it for the two primary ones I have. Can anyone think of any possible fixes or traces for this? A cursory check didn't indicate any console commands that would help, but I wouldn't be surprised if I missed something.

Have you any mods ???

Have you tried to verify the Integrity of the game Cache files ???

https://support.steampowered.com/kb_article.php?ref=2037-QEUH-3335

The cause was probably a mod, I'll detail below:

It turns out that the problem was a custom animation I had in place for my unarmed power attacks. I lost track of which animation mod I was using, but I don't think it was too relevant for the purposes of the bug trigger. Basically, removing the specific animation for the power attack that wasn't working (in this case, hand to hand left), completely fixed the issue. After removing it, saving, and trying to add the same animation again, power attacks costed nothing again. However, after adding a custom animation from a different unarmed combat mod, it turns out that one works. Huh.

Anyway, I still have no idea what actually caused it to occur in the first place, but probably this would work for anybody who is encountering this issue with their power attacks. If you don't know exactly what the cause is, try removing any corresponding custom animations first.

It is notable that I am using FNIS for PC-exclusive animation paths, and the reason it worked on a new character is because my test race was default Redguards. This is what tipped me off; making a new character with a custom race had the same animations, and thus the same problem.

EDIT: I had to do a little digging, but I confirmed that the problem animation was from the http://www.nexusmods.com/skyrim/mods/9421/? mod - specifically, it was the Round Kick, which is the right-hand power attack by default. I set mine to left since I was using animations from that and Unarmed Warfare. Presumably, using this with FNIS caused problems because of something specific to that animation; the author of FNIS speculated that it may have been a bit shorter than the default or something.

Curiously, though, I could not replicate the problem again after switching my animations around initially. Don't know why. Skyrim is weird.