Hi!
After travelling between overworld and nether the heart container hud does not show the additional hearts generated by the spectrit armor.

I have the same problem too. Fortunately, however, all you need to do is take damage or heal yourself and then the HUD will correct itself.

This is covered in known bugs. It is a desync between the client and server that happens because the Player is recreated on the client when travelling to another dimension. It is a Vanilla bug, though you won't usually see it because Vanilla has no passive effect that extends max health. I will leave this open though in case I eventually find a workaround.

Vanilla bug report: https://bugs.mojang.com/browse/MC-29386