In this example I exited a mind control with my power word: shield hotkey (right in the middle) and as you can see it shows all grey. The button is not usable. I MUST at a minimum, completely log out to the char select screen, and back in, to use it again. I believe this is a long standing issue, but I never knew what caused which spells to be rendered unusable. For example, in BWL on my warrior if I were mind controlling razorgore and used a spell to exit out of that, it would do the same thing. But, I never realized how it was determined which button becomes unusable until now.

Just retested, still a thing.

Ive also found that CONSISTENTLY if a mob has an identical spell name as something I already have, Bartender disables those spell on my hot bars and I cannot use those spells until Ive relogged. For instace, if I mind control a Cabal Shadow Priest, my Mind Flay and Shadow Word: Pain become permanently disabled.

https://tbc.wowhead.com/npc=18637/cabal-shadow-priest#abilities;mode:heroic

Its not listed here but they have a mind flay that ticks for about 2100 damage.

Here is an example https://youtu.be/4blpqtJpPgE
You can see at 0:20 as soon as I complete my MC cast those two buttons go unusable. You can see at 1:07 a UI reload does not fix the problem.