When a light-emitting fluid such as lava is placed in a cauldron, no light is produced by the fluid. As well, the texture has no emission, and so will be darkened like any other block in dim light.

Closing this as not planned for 1.16. 1.18 made each fluid its own block thus making them glow is trivial.

I am not sure if there is anything I can do about this, but I can look into it. One big detail is cauldrons are a block model rather than a tile entity, so I cannot make the fluid texture "glow" properly. I can look into making it emit light though as I am already triggering block updates when you change the fluid.

I just tested this and it appears the game does not like me having the light level based on the TE, despite sending block updates. I will have to look into this more at a later date but for now this is "can't fix"