It's not a major bug, but it's a bug none the less.
Cheers!
I've had that happen, but I think it's when that dweller was already leveled up during the objective.
I think each dweller can only count once towards the objective.
It's a bit annoying when you have to level up 50 dwellers and your population is near maxed.
Bring them in from the Wasteland. Send them to train for 24 minutes. Send them to die in the Wasteland. Repeat.
As stated somewhere else in this forum, when objective has dwellers (lowercase d) it needs x different dwellers to complete. When it is Dwellers (uppercase D) it could be the same ones).