Photon Geyser should have done much more damage here:
https://play.pokemonshowdown.com/battle-gen7balancedhackmons-722277945
Because of the Burn, my SpA was higher than the Atk, so that Photon Geyser was a special move - and according to the calc, it should do around 50% of damage or more, coming off of Slaking's SpA. However, it did only 31%.
This is the correct behavior.
Since it cape up in discord: Burn doesn't actually drop your raw stat, which means that in this same case, the burned 300 Atk Necrozma still uses its attack stat for Photon Geyser over the 200 SpA, and as such deals damage coming from the Atk stat, which is then halved by the burn.
e: To elaborate, ever since gen 3 burn's damage reduction has been done as a modifier to the damage formula, not as a raw stat reduction to Attack. That's why you'll see that the damage for these two calcs is slightly different.
-2 252+ Atk Landorus-Therian Supersonic Skystrike (175 BP) vs. 0 HP / 0 Def Paras: 1096-1296 (519.4 - 614.2%) -- guaranteed OHKO
Possible damage amounts: (1096, 1108, 1120, 1140, 1152, 1164, 1176, 1188, 1200, 1216, 1228, 1240, 1252, 1264, 1276, 1296)
252+ Atk burned Landorus-Therian Supersonic Skystrike (175 BP) vs. 0 HP / 0 Def Paras: 1098-1292 (520.3 - 612.3%) -- guaranteed OHKO
Possible damage amounts: (1098, 1110, 1122, 1136, 1148, 1160, 1176, 1188, 1200, 1214, 1226, 1238, 1254, 1266, 1278, 1292)