Usually when this happens to me, its because I keep spamming 1 (Sinister strike/Ambush) and then it registers Sinister Strike before it enters the Stealth "phase"
So I end up casting Sinister strike instead of Ambush
But all I do really, is just try to stop pressing anything and then wait till ambush becomes available.
Don't really have a fix for it
Having a separate key binding for Ambush will allow you to spam it without accidentally triggering whatever the button shares with your non-stealth bar. Easiest way to land fast Cheap Shots/Garrotes in PvP too.