Copy/pasting from a prior post as it was asked to be reposted if found in a newer version, and the person who wanted me to post this said that it is still present in 8.2:

Seemingly, physical Light That Burns the Sky (as in, the user's attack stat is higher that its special attack stat) ignores attack boosts of the user.
When testing it against an opposing Solgaleo, there seemed to be no difference in damage when used at +0 attack and at +6.
the damage calcs should look like this:
220 Atk Necrozma-Ultra Light That Burns the Sky vs. 252 HP / 8 Def Solgaleo: 88-104 (18.4 - 21.7%) -- possible 5HKO
+6 220 Atk Necrozma-Ultra Light That Burns the Sky vs. 252 HP / 8 Def Solgaleo: 354-416 (74 - 87%) -- guaranteed 2HKO
the latter of which was nowhere near that high ingame.
I believe this either because of:

a) physical Light That Burns the Sky just ignores attack boosts in general
or
b) it is always special regardless of the user's attack stat being higher than its special attack (which would explain the pitiful damage it did even at +6 attack)

Thanks for the report! I've been able to reproduce this. Light That Burns the Sky uses its special stat when it shouldn't.

Hi there! We're currently working through older tickets, and upon re-testing this issue, it seems to still be present in the current version (8.3.6).

My method:
1) Gave myself an UltraNecrozma with the necessary ultranecrozium z, lvl 80, with Photon Geyser.
2) Gave an NPC Trainer a Solgaleo (following the above example), lvl 80.
3) First test: Gave Necro 0 evs and ivs, except 252 spatk evs, and 31 spatk ivs (stat was 312 total spatk, 312 atk) to test special attack Light That Burns damage against Solgaleo - attack did about 67%.
4) Second test: Gave Necro 0 evs and ivs, except 252 atk evs, and 31 atk ivs (stat was 347 total atk, 244 spatk) to test physical attack Light That Burns damage against Solgaleo - attack did about 73%.

My findings: It does seem to be using the special attack stat in all cases, rather than using whichever attack or special attack stat is higher.

As such, I'll go ahead and move this to internal. Thank you!