Bongos: Dissappearing Abilities (Envenom)
 

Whenever I enter combat Envenom seems to disappear off my bar, but only from Bar 1. If I move it anywhere else it stays there just fine.

Anyone else have this problem?

Go into Bongos settings and select "Show Hidden Buttons".

There is a bug where when you switch stances in combat buttons that are hidden in one stance will not show in another stance. In your case, I'm guessing Envenom is on a button that doesn't show while stealthed. When you come out of stealth the button remains hidden.