Note that custom servers can change hardcore mode via Join packet.

Yea, low prio issue for now, since this WIP release has way bigger issues.

If I had to guess this probably happens because the cached render stays from the previous world, taking damage seems to reset it, and it also works both ways. (Going from Hardcore world with hardcore hearts to normal world keeps the hardcore hearts.)

A proposed solution would be to reset the cache on world exit or on world join