(There is some internal error on Mojira, so I cannot perform a search. Sorry if that has been posted already.)
1. Eat a Golden Apple
2. Get Health Boost / 2 additional hearts
3. Wait for Health Boost to wear off, your heart container amount drops from 12 to 10 (which is normal)
4. But then the "filling" of 2 of the 10 heart containers get taken away from you.
Basically: Let's say after you ate the golden apple, you had enough hunger so your temporary 12 heart containers, and not even your 10 heart containers get filled fully.
Let's say from the 12 heart containers you have while the Health Boost is in effect, only 7 are filled.
After the Health Boost wears off, the heart amount drops from 12 to 10, which is fine - but instead of keeping at least your 7 out of 10 filled heartcontainers, you hear the "hurt" sound, and you get 2 additional filled heart containers taken away, so you remain with only 5 instead of 7, which should have remained.
You can see it better if you have enough hunger, so eating an apple (= heals 2 hunger chopsticks) will not make you regen all of your hearts totally.
Of course you should have like 4-5 hearts only before you eat the golden apple - as I said, it is better to see.
If you are testing this with all 10 heart containers filled, you might not clearly see this bug.
PS: I recorded it - in case it hasn't been reported before, I will cut and upload it for you as proof.