[Autumn]

It's not out-of-character information to notice someone is affected less by healing magic than others being healed as well.

If we were in a club, and all club members had their left arms broken (for the tax benefit in this hypothetical county), and someone broke into our club, cast healing on all of us, and all of our arms were healed except yours, I'd be wondering why. If there was a in-universe reason for that (for example, the knowledge that homunculi heal less than others do) then it wouldn't be a jump in logic to assume that's why it happened.

I really can't see how it's metagaming for people to see what is happening with their IC eyes infront of them and make assumptions based off of their in-universe knowledge, especially when there can only be so many situations of what could cause said lack of healing.

- (Would healing a vampire and having the effects be lesser be something you can notice?) It would probably be noticeable, although there are other reasons that the magic would be less effective. Strong magical resistance, environmental reasons, maybe the priest is having an off-day, wounds that seem more severe than they actually are.

This much is true, sure. And it would be more convenient to you if your intention is to point out who and what heals for less HP is not what it seems, so how about I put it like this instead. 

The above mentioned chat log still would lie to everyone else in the fight like my original suggestion, however the person who healed the Homunculi or Vampire would get an additional message printed out in their chat log something similar to like: "Something felt a little off there." or "The restorative effect wasn't as effective as it seemed."

This message only being visible to the healer would solve the metagame-like knowledge of everyone else in the fight becoming aware of the Homunculi or Vampire instantaneously, while granting the healer the avenue in which to determine whether or not they would want to confront the Homunculi or Vampire in IC, whether in Secret or by blatantly exposing them.

Is this acceptable?