If a client is off the ground for an extended period of time (more than a double jump would give), anything held or flicked by that client will slowly lower its relative position before resetting to neutral position (and starting the cycle over).

The most visible example of this is when a player is stunlocked:

But it also happens when modding to fly, so it's unlikely that the bug is related to being in knockback.

This bug is very impactful in gameplay when it comes to client flight, as no flight on client can seem to last longer than about 10 seconds, regardless of how stable it is:

(These might be separate bugs actually, but either way, flick and hold break when in the air too long on client.)