It's to do with pressing releasing shift before or after releasing any of the wasd keys.

So, you only DNP wasd but you do post the shift variants. If you then release shift before releasing any of the wasd keys, your alts will not register the release of that wasd keys and continue as if the key is still pressed.

The trick is to either DNP none of the wasd keys in all of its variants, or just to get used to always stopping moving your alts. So you drop the wasd key. But somehow that feels unnatural (at least to me). So I just use other buttons to move my alts now. :P It almost needs a conscious descision for me to release the wasd key before the modifier key. :P