The game doesn’t adequately signal that sinking in lava while fire resistant will become fatal given time. There are a few possible solutions depending on how much the model of “dissolved in molten lava” is changed:

• Messages involving sinking in lava should indicate more clearly to the player that even though they are fire resistant, they will eventually die by sinking below the surface and dissolving. Possibly separate messages when the timeout is getting low, such as “You are beginning to dissolve.” and “You are dissolving faster.”
• Sinking in lava while fire resistant is fine, and doesn’t kill the hero if they have magical breathing. The death that occurs if they don’t is changed to “drowned in molten lava”. This could mean the hero can swim through lava so long as they retain both fire resistance and magical breathing.
• Sinking in lava dissolves parts of the hero’s body as they go deeper, causing progressively increasing amounts of HP damage, probably fractions of maximum HP, which eventually go over 100% as they slip fully underneath and kill them outright even if they’ve been healing the damage every time. Messages would also need to be updated.