Hm, I may have a bug here. I used the map editing tool to make Misdreavus (along with a few others) temporarily available in an early route. I ended up using her to deal with Brawly. I lead off by spamming Fade (+1 Evade/Speed) 6 times for +6 stages each. Strangely, I was able to keep using the move to buff stats to what would be +10 stages, though I didn't go further to find out if I could use it the full 15 times without being told it can't increase my speed or evasion higher.
So yeah, sounds like a near-god-like mon to deal with Brawly, what with having Psywave, Astonish, and Dusk Shield, right? I got to the Hariyama and after lopping health off at ~1/3 a hit and then watching him heal via Sitrus and Super Potions with a Belly Drum thrown in there, he started using Dig. Now, I should have at most a 30% chance to be hit, if I understand evasion properly and am using the right gen's data, but the guy nails me with Dig. I think 'Alright, fluke I guess, just use a Potion so she survives a potential second hit, as unlikely as it is.'.
The guy managed to hit every single time he used Dig without exception, as if I didn't have evasion buffs. We went through a cycle of potions and Psywave vs dig and super potions till I brought 'em down, since Brawly (presumably) ran out. He absolutely never missed, and as far as I'm aware, they don't have No Guard or similar. I even save-scummed to start that fight over again for more tests and did further testing to see if I could manipulate the RNG clock by switching up choices to try and force a 'Miss' result. Nothing but hit after hit.
I have no idea what's going on here. Maybe evasion is bugged. I went out to test Fade against wild mons outside afterward and it appears evasion just doesn't work, though I'd test with other evade buffs. I seem to recall using Smokescreen to an actual effect with Cyndaquil, though I'm not certain and he doesn't remember it anymore so I can't go test it.
TL;DR, evade and possibly accuracy appear bugged out and non-functional, or close to it.