If it is a normal disconnect I think the game usually do notify us and I assume correct itself, but if it is a bugged disconnect I am not sure.
I can say that I have a problem with that when I try to level up my characters my game sort of crash Internet wise I suppose, since as if I try to do any online battling it just stops and says loading or try to gather rewards from the Mailbox I usually don’t actually receive them and have to close and reopen the game again to actually claim those rewards. I think it also bug when I try to claim my free 60 EXP bonus and might not actually register when I go to the campaign.
Been a while since I really had to wrestle with those problems so I don’t 100% remember, these days I know not to do anything else than try to level up and then close the game again and reopen it to try and level more.
I don’t know if you are in an similar situation of some sort even though I think it isn’t exactly like this as you were just leveling up rather than trying to level up characters, but it reminded me a bit and thought I mention it as it is so something about if it is that the game bugged like I had there. Likely something else, but yeah it could be the case of that something caused the stamina from the level up not to be properly counted and as such kind of disappeared.