What you're doing wouldn't really confirm a bug. I wouldn't necessarily expect to be able to get the highest increase successively. If you did the test, going from level 1 to 3, 20 times, and saw lone 12s going from 1 to 2 and from 2 to 3, but never for both level-ups on the same run, that's likely perfectly normal.
I don't know what the stat growth probabilities are for this game, but HP is probably not weighted to the high end. By that I mean that getting +12 may be the least likely outcome by a fair margin. In that case, what you're having isn't really bad luck exactly. You're just failing to have a run of good luck.