This behavior was introduced in 25w19a.
Steps to reproduce
Place a cauldron 2 blocks above the ground.
Fill the cauldron with lava.
Go into survival mode.
Stand directly underneath the cauldron and jump.
Observe that you are now on fire.
Observed behavior
The bounds of cauldrons now seem to extend to the very bottom of the block, as opposed to above their feet, applying the effect of the substance they're filled with when contacted from the underside.
Expected behavior
Cauldrons would not apply the effect of the substance they're filled with when contacted from the underside, as was the case in versions prior to 25w19a.
Linked issues
is duplicated by 7
MC-298863
Entities get effects from the liquid in a cauldron when hitting the bottom of it
Resolved
MC-299134
Lava cauldrons burn baby mobs from below
Resolved
MC-299447
Lava cauldron can burn entities not touching the lava
Resolved
MC-299622
Standing below the gap near the legs of a cauldron with lava will affect you the same as being inside of it
Resolved
MC-299659
When you touch the bottom of the cauldron,you will be burnt.
Resolved
2 more links
Can confirm. Also this isn’t exclusive to the lava cauldron. the water cauldron is also affected, you can use it to extinguish yourself via the same manner,