The lighting of magma block is incorrect on some sides.
While the magma block is surrounded by blocks the most west facing block has no lighting on the top face of the block.
When placed in a line (not surrounded by blocks) the sides of the blocks the magma patch wont have lighting.
I have attached 4 images showing the issue in 1.21.5 Pre-release 2, and 2 images of version 1.21.4 showing how its supposed to work.
Best seen in dark areas/at night.
In daytime or in brightly lit areas its hard to see the unlit sides.
Also less visible when the brightness is higher.
I am able to reproduce the reported behavior. It was introduced in 1.21.5 Pre-Release 1.