had it happen to me too, Orlash and a frozen/thunderstom pack, was "only" lvl28, still took a while to kill, glad it wasnt HC, died 4times ^^
And, the pack, was it in a nearby area, mine came out of the next room i was in as boss spawn, was in A3 keep layout, same as u. Perhaps this exact layout is slightly bugged
For those of you that are unaware, spawning the Rift Guardian in a Greater Rift will despawn all enemies in a certain range of the Rift Guardian spawn. It does not, however, despawn every enemy in the rift. If you move further into the rift, you can and will run into enemies. So, you know, don't drag the RG into parts of the map that you haven't explored yet unless you want some extra trouble.
ffs
And, the pack, was it in a nearby area, mine came out of the next room i was in as boss spawn, was in A3 keep layout, same as u. Perhaps this exact layout is slightly bugged