As the title suggests, guess that the terrain judgement of move: Grassy Glide causes damage before the terrain cover takes effect.
Have a small test:
The enemy send out Rillaboom(Tapu Gorilla, lol) and I send out a whatever pixelmon. In the first turn I change my pixelmon to Tapu Lele, as we all know that the Psychic Terrain will cover the Grassy Terrain and the priority of Grassy Glide will become normal then it can be used naturally. But the enemy's Rillaboom fail to use Grassy Glide then.
I'm not sure what's the source of the problem, guess it's cuz of the judgeement of Grassy Glide. Hope that U could have a check and solve it breezily. :)

Tested on Showdown and it seems to go through as a normal move.
Was able to replicate the error in 8.1.2

Steps to Replicate, Give NPC A Rilaboom with Grassy Surge Ability and Grassy Glide Move.
Enter battle with any poke, and switch to Tapu Lele.
Grassy Glide fails as it was thought to be a priority move(blocked by psychic terrain) but the terrain was switched and grassy glide is no longer priority.

The error seems to be in the unimplemented speed mechanics of this Generation where speed other things are recalculated mid battle. This I believe has not be implemented in Pixelmon hence Grassy Glide hence this error.

The ticket will be awaiting move to the internal tracker thank you for reporting, please keep a look out on the ticket for updates.

Moved to Internal. Keep an eye on the ticket for any updates!