Sorry for the bump, but I've figured out what it is - it works exactly the same for all projectiles, including perfect-guarded reflected ones. Basically, when a projectile is fired, it takes the game half a second to realise the projectiles exist, and therefore it takes half a second or so to apply collision detection.
The reason this works with reflected projectiles too is because (I think) the game creates a new projectile, rather than changing the direction of the current one. I have concluded this from the fact that a reflected projectile will last for the same amount of time after it has been reflected that a non-reflected projectile would. Furthermore, if a reflected Wisp projectile has no enemies to home onto, it disappears when perfect guarded. Although I think reflected Wisp projectiles keep their tracking timer, eg. if the projectile is no longer tracking the player when reflected, it won't track the Wisp that shot it again.
Either way, any newly spawned/reflected projectiles can travel through walls for a brief moment because the game can't keep up with them.
