It does seem to get buffed to 60 BP when I deselect Grassy Terrain: '252+ Atk Choice Band Tera Grass Rillaboom Grassy Glide (60 BP) vs. 0 HP / 0 Def Mew: 210-248 (61.5 - 72.7%) -- guaranteed 2HKO'. You can see that yours has the 'after Grassy Terrain recovery' so the terrain must be active if...