I am playing on a friend's server who hasn't changed the default mod settings at all, that's said
I made Yellow Heart Canisters (5) a few days ago,
These work just fine.
Now I crafted my 6th Yellow Heart, and I am shocked.
The 6th Heart didn't stack with the other 5 ones.
I strongly believe it doesn't stack with the others cause it got crafted later, which also means I probably didn't use the same crafting recipe as before.(shapeless)
Maybe you fix this in the future.

Thanks for listening.

~DCE

Edit
Playing on Allthemods8 Server with a lot of other Mods.
The Server Owner also made no updates to the server at all.