there is a bug with the Stamina when you fight the Echo Knight.
i dont have anything crazy but for some reason when i fight him i ended up with 1596 stamina.
i chose some stamina regeneration echos or reductions , and i used the the Stamina Wellspring skill , and when i click on Vial of stamina i ended up with that large amount , but this only happens when i fight with the boss , after that or inbetween the fight if i run out of the buff it resets
thanks for sharing, that indeed does sound odd. i will change the category to wicked bug reporting, so it is in the right place 
ohh now it happened even when i fight with other enemies , so it basicaly have something to do with the fight ,
i press the Stamina Wellspring skill , then i hit the enemie and then when i use the Stamina Vial - the bug happens
nice catch! and thx for additional info, i think that helps. not sure if this helps here too, but from other bugs, the dev team requested player logs etc.
can you maybe add the reports as well. how to get them is described here: How to Properly Report Bugs!
ohh , it can be done in the overworld too , this absolutely eliminates the stamina from the game
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well now iām interested^^
there is something with which vile you are using , for the small one it only gives 550 ( which is more then enough ) but still
550 seems already a bit much for the small one lol i never use those, i just sell them, because i have stamina regen/on hit on my weapon. thats enough.
can you please check if u can link your logfiles as well (se my previous post)? you narrowed it down pretty well, but i think the logs can at least not hurt while investigating the issue
all in all it sounds broken to me.
EDIT: it sounds like stamina vials do not refill your stamina up to max, but rather add stamina on top, as if they would increase max stamina. wonder if u can stack that, what happens if u chuck 3 potions? 
just confirmed
only did stamina wellspring, and then took a medium vial
only last for the wellspring buff duration
thanks for the report