Why does WASD movement seem unable to cancel an auto-attack?

The observation alone does not show whether this is intentional or a control bug. One possibility is that the two movement methods handle a move command during an attack differently. To narrow it down, compare the same attack timing and settings with each method, and note whether WASD is ignored until the attack begins or until the whole animation ends.

Either way, being unable to move during a slow attack can be rough. If you need to get out before the hit happens, mouse movement may be the safer option for now. A short, repeatable comparison of the two controls would also be useful feedback for the game team.