This is somewhat anecdotal, but it seems like my characters always get the same hit points on level up. I verified that the relevant difficulty checkbox is not checked, and, anyway, it doesn't seem to be max hit points every time, just the same hit points every time. For example, my wizard with a +1 con bonus has leveled up 5 times and gained 6 hit points each time, corresponding to a roll of 5 on the dice each time. I've seen similar for each character. My cleric gained a point of con at level 4 and now gets one more hit point per level than he did for levels 1-4.
Again, may just be sample bias. I haven't put in the effort to test this properly, but it seemed noticeably strange to me.