There have been a few bugs where certain specific skills had bugs with Multistrike which caused them to not gain the damage bonus (e.g. Lightning Strike projectiles), or consume too many charges (e.g. Flame Dash). Basically all of these bugs have already been fixed.

But there has been some confusion about how well Multistrike has been working. Of course, generally speaking Multistrike is supposed to allow you to cancel out of it with other skills or movement, but afaik it has never "self-cancelled".

SilviteRamirez

with other skills or movement

So what about holding down the mouse button with Flickerstrike? Can you prematurely cancel the third strike this way?

If you are just holding down Flicker Strike, and not doing anything else? No.