If you use the looking glass while walking and then go out of it, you will continue walking until you press the same walking key(s) again. Using the looking glass a second time also stops it.

Version: 1.16.5-1.13.12

Pretty sure this is because MC doesn't handle not receiving keypresses well when something interrupts it, like the various ways to trick it into thinking a key is held-down when it no longer is.