I was outside Rhaz Inkur.  Grouped with a lvl 44 bard, 49 paladin and a 50 ranger.  We were killing the 4 dot effreeti, and various other 3 and 4 dot mobs in the area.

For those that this is happening to, try using a lower level ability.  Bounless fist VII might be locked out, but boundless fist VI might work.  I created an alternative hotbar for now with the next lower set of abilities for those that are getting locked out.  It saves from having to relog.

My Monk is a Level 49 Drunken Monk on Xeth.

I've noticed that sometimes when I feign, some of my abilities become unusable.  I get a message "sorry, it seems that reaction has timed out"  It is odd, because It only affects the highest level for those abilities.  So far I have noticed this on Staggering Punch, Ashen Hand, Crescent Kick, Boundless Fist.  At some point it fixed itself, maybe when crossing a chunk, but happened again shortly after.

This may be a UI issue, I use Core UI with the latest update, but it doesn't seem likely to me.