While being under the guard of another character, and by being not even 1 step away from him, it appears that -guard doesn't guard or when it does, it comes with a huge delay. That seems to happen often with monsters that have extended range like wyverns, or when the number of monsters exceeds four attackers.
NB: I haven't been triggering any attacks of opportunity and was not being targeted by summons (I know these two things bypass the guard).