Happened to us today as well. respawned after clearing an objective, a bomb guy spawns next room and runs in while we are still in the middle of our "checking the gun" animations and blows everyone up...
I also started a topic (well, as soon as the mods finally approve my posts) about enemies spawning to cleared buildings being super annoying.
Best would be to tag a building as "cleared" if a player has been in every room with no enemies in the building, then not allowing any enemies to spawn in that building for as long as the players are within X meters (and in general not spawning enemies within X meters and not within LOS even if farther than X ) and for actual cleared checkpoints not allowing enemies to spawn within X meters of them at all once cleared.
This time it wasn't a suicide bomber since security doesn't do those, https://youtu.be/RgEXrkEp0xU
The spawn zones should be protected somehow even when playing with randoms that are so high-speed-low-drag that they don't bother to clear buildings when advancing and thus leave plenty of enemies to come at them from the rear...
Of course having enemies spawn in recently cleared buildings makes clearing them slightly less useful than it would be in the real world.
edit: edited for clarity.