I'm not entirely sure if this is how it's supposed to work, but this is normal? At least from what I know. All other times I've had one in my inventory on other servers this is how it worked as well.
You see one little heart for one box that you buy, and one big one for 5. Other players can see it as well.
Okay, bought 5 hearts, it didn't turn big and i only see 1 little heart still and only 1 heart shaped box in my inventory, but if i drop it, there is still a heart shaped box in my inventory, suggesting that there is 5 there.
Then they must have one big one for 10. I know that it makes a big heart for a certain number of hearts, just can't remember what number it's at