Strength of the Ages Mastery Only Gives +10hp for Monsters
It's pretty easy to figure out the issue from the title... the mastery is only giving +10hp for Large Monsters but it's supposed to give +20 according to the information about it.
There's also an in-game tooltip issue as well. When you hit tab to see the team's builds and summoner spells (where it now shows which keystone mastery each player chose) the wording makes it seem like you'll be healed for 100hp from large monster kills as well (which doesn't happen). If you pull up the masteries page it says that only siege minion kills will heal for 100hp after capping the hp bonus but it's worth noting that the in-game text is incorrect.
I have yet to test whether or not this issue happens in all games or just my last one. If for some reason the error is champion-specific I was playing Amumu jungle when I noticed it.