This happened when I updated to Tinkers Construct 1.8.0 (Beta) on my Direwolf20 1.7.10 server. The author of Tinkers Construct said its not their fault. And AFAIK Botania is the only other mod that uses the extra hearts.

Screenshot: https://www.dropbox.com/s/uui6cbwe8l9wc3h/2014-12-19_13.50.26.png?dl=0

I'm using Botania R1.3 143.

The hearts are vanilla behaviour.

With just Minecraft 1.7.10 and Tinkers Construct 1.8.0 it works. So it's not a vanilla problem. So who is to blame then? Maybe Pams Harvestcraft?

Yes, the last part was sarkasm.

All I do is make a normal item with a HP bonus attribute, you can do that in vanilla.