You were right about the HP update delay. The game engine was acting on the correct value, but the UI was only ever updated when HP was lost. So if you were down to 4 HP, then you fully healed, you would appear to still only have 4 even though you did have 10 until you lost -1 HP, then you would visually jump from 4 to 9.
I fixed this and other things now that the jam is over in what I hope becomes a trend of actually continuing development on a project. I welcome any other feedback you have, and thanks again for playing!
https://applepieforge.itch.io/whack-a-zombie/devlog/878850/v001-posted