In sun/electric Terrain, pkm with Protosynthesis/Quark Drive will lose their boost after mega. But it is still showed with no effect.
https://replay.pokemonshowdown.com/gen9mixandmega-2120837943-lg4zaxv4fww8zppsp5t2558tq11nulfpw
This is my test. Pkm has a spe boost before mega, because it is...