Potion dont prevent burning...
It only halvens the amount of damage you get from fire ... and as Niara said it dont stack ...
So if your character allready is resistant to fire damage it does nothing and that is corect by design.
So if you stand in fire you get 1d4 fire damage + you can get Burning status that gives another 1d4 per turn ...
If you stand in fire with fire resistance (no matter if from race or potion) situation is same but your damage recedived is reduced to half ... so 1d4/2
And if you havw racial resistancw AND potion they dont stack so still 1d4/2
With resistance firw still burns only slower.
The potion description doesn't say you only take 1/2 damage from fire. It Says "Gain resistance to Fire damage until your next Long Rest" whereas with the Waste Land wander Fire it specifically states that you 1/2 damage from fire. Now you can see how someone can be confused by this and it should be corrected in the potions description.
Because pretty much the only way to save that guy is by pure luck. It took me several loads to save that guy.
