Only seems to be one mention that I can find of foul play not working properly as a forte but it was never addressed. After some testing, it seems the issue is that it probably uses the target's attack stat as intended, but it ignores the target's stat stage changes to attack, which doesn't happen when using foul play under normal conditions, in which it does account for stat stage changes to the target's attack. Example from testing: https://replay.pokemonshowdown.com/gen9fortemons-2464042614-gu8i14gyycs8720mgj15km2ouvck99ypw