Not sure if this is just me, but I have been having an issue with lower level monkeys that stop gaining experience and won't level. This has happened to me at least ten times with male and female monkeys of varying levels. What happens is this: Tame a monkey, its sits on my shoulder and I do a bunch of stuff. I set it down to apply its level points and once the monkey reaches lvl 8, the experience bar remains full but there are no more level up points for me to place and the monkey stops gaining experience and won't level up.
I was wondering if anyone else had this problem, or if I'm just super lucky.
--EDIT-- So as of 6 January 2019, this leveling glitch has been cleared up. Thank you devs for your quick handling of this and many other bugs!