Endless mana potion CD timer delayed starting after using

When I use the Endless mana potion the cool down starts randomly after I use the pot. Sometimes it starts the cool down instantly like it should, some times 10 seconds later, it’s really random, and sometimes when I’m drinking to mana back up, it’s like when I hit a threshold of mana it starts the cool down, delaying the natural cool down timer making the cool down timer longer than it’s suppose to be.

Anyone know if this is a bug? or is there some thing I’m unaware of?

EDIT: I’m noticing a problem with being in combat forsure.

The only time the cooldown doesn’t start is if you use it while you’re in combat because of potion sickness.

If you use it out of combat but the issue is still there then it’s a bug.

Potions can only be used once when in combat.

The cooldown will not begin until you leave combat.

Out of Combat:
Use potion.
1 Minute cooldown begins.

In combat:
Use potion.
Combat ends.
1 Minute cooldown begins.

Thank you both, it’s small delay out of combat. like 5 seconds after I leave combat I’m drinking and 5 seconds later is starts, that normal?