I have reported this through the game, but I thought I’d also post here with the rest of my findings:
I was running a forge guard build with shield rush specced. At lvl 35 I respecced to another build which didn’t use shield rush. I also changed gears and went for a 2h weapon (no shield).
I left shield rush in the same spot in the hotbar. When I tried using it (didn’t even remember I don’t have a shield anymore) it just kept running in place and the only way to stop it was to cast another skill.
Now, I know I need a shield to use it, but if I don’t have one it should be disabled in the hotbar, rather than do that weird visual thing which made me think it was a bug caused by the respec rather than simply not having a shield.