When the player is fully submerged in lava, no air bubbles appear on the HUD above the hunger bar, which implies that players can breathe in lava.

(I also included a screenshot when the player is in water for reference) 

How to recreate:

Mine under a lava lake

Mine the block directly above you (And lava will flow on you).                                                                   

Observed results:

No air bubbles appear on the HUD (Which means players can breathe in lava)

Intended results:

Air bubbles should appear above the hunger bar, in the same fashion as the player is suffocating.

This bug is evident in my version 1.20.4, but also affects 1.21.