If you speak about druid it’s not a bug, watch their positioning they can’t drink while already in stealth so if they move they have to get out to redrink
Druids can shapeshift and go stealth while eating/drinking. Night Elf druids can drop out of combat via Shadowmeld (Racial), otherwise you have to wait to get out of combat then run into cover, start drinking and pop cat stealth.
This is in the game since like vanila, Rogues and droods were always able to start eating/drinking and go to stealth.