Steps to reproduce:

1. This bug is somewhat dependent on render and simulation distance. From the values I tested, 32/32, 16/16, 8/8, 7/8 and 8/7 produce the bug, while 32/5, 2/5 and 2/32 don't. Best choose 32/32.
2. Make a single biome (desert) world with the seed "1566450010406945717".
3. Enter spectator mode.
4. Execute "/gamerule doDaylightCycle false" and "/time set midnight".
5. Execute the following command: "/tp -2500 100 0".
6. Fly in the positive Z direction until you see a submerged ruined portal at approximately -2500 670.
7. The lighting around the portal is broken; one half is completely dark and the other is bright, separated by a chunk border; furthermore, there is light on the water surface that is missing a source.

Note that merely teleporting to the ruined portal's coordinates does not produce the bug.
Edit: This bug exists since at least 1.18.2; before 1.18.2, the ruined portal was not submerged and the bug does not occur at these coordinates, so I have no way to tell when it was introduced.

Second found example:

1. Choose render and simulation distance 32/32.
2. Make a single biome (desert) world with the seed "3387133703615993292".
3. Enter spectator mode.
4. Execute "/gamerule doDaylightCycle false" and "/time set midnight".
5. Execute the following command: "/tp 0 100 390".
6. Fly in the positive X direction until you see a submerged ruined portal at approximately 330 390.
7. The lighting around the portal is broken; one half is completely dark and the other is bright, separated by a chunk border; furthermore, there is light on the water surface that is missing a source.

Third found example (in a standard world!)

1. Choose render and simulation distance 32/32.
2. Make a standard world with the seed "-7140106853293537955".
3. Enter spectator mode.
4. Execute "/gamerule doDaylightCycle false" and "/time set midnight".
5. Execute the following command: "/tp 8965 100 9000".
6. Fly in the positive Z direction until you see a (non-submerged!) ruined portal at approximately 8965 9300.
7. The lighting around the portal is broken; one half is completely dark and the other is bright, separated by a chunk border.

Fourth found example (non-desert portal)

1. Choose render and simulation distance 32/32.
2. Make a standard world with the seed "-8564942748222529924".
3. Enter spectator mode.
4. Execute "/gamerule doDaylightCycle false" and "/time set midnight".
5. Execute the following command: "/tp 27585 100 28500"
6. Fly in the positive Z direction until you see a (non-desert) ruined portal at approximately 27585 28825.
7. The lighting around the portal is cut off along a nearby chunk border.