The bug
1.18 pre-release 1 fixed cauldrons (MC-240021) by removing the cullface arguments from many of their internal faces to make them visible for players inside. This fix works for normal cauldrons, however cauldrons with contents still have the bottom face be culled in such cases, which is undesirable.
How to reproduce
Refer to MC-240021 for a reproduction guide - the only difference here is you should fill the cauldron with water before entering it.
Expected results
The bottom of the cauldron would be visible.
Actual results
It is not.
How to fix
I would HIGHLY recommend just copying the model json files over from the resource pack attached to MC-240021, as they fix this issue entirely, as well as fixing MC-205095 as an added bonus.
Can confirm.