What type of bug are you reporting? Mechanics
What is the bug?
When a pure Flying-type Pokemon that also is affected by Forest's Curse / Trick-or-Treat uses Roost, it becomes Normal/Grass or Normal/Ghost type, respectively. This is similar to how using Roost as a pure Flying-type will cause the Pokemon to become Normal-type.
Are you able to replicate it? If so, how?
https://replay.pokemonshowdown.com/gen9doublescustomgame-2169810217
Drain Punch should have been super-effective, because Corvisquire should have been Normal/Grass type.
What is the bug?
When a pure Flying-type Pokemon that also is affected by Forest's Curse / Trick-or-Treat uses Roost, it becomes Normal/Grass or Normal/Ghost type, respectively. This is similar to how using Roost as a pure Flying-type will cause the Pokemon to become Normal-type.
Are you able to replicate it? If so, how?
https://replay.pokemonshowdown.com/gen9doublescustomgame-2169810217
Drain Punch should have been super-effective, because Corvisquire should have been Normal/Grass type.