It's my map and though I have seen it played for only two rounds I think I understand the problem. The routes towards the flags are all quite fast, the fastest one is just above 30 seconds, the underwater tunnel. The defenders all spawn near the flag, which gives them respawn timer + 5 to 10 seconds walking as their effective time to flag.. According to that it is balanced, as long as the attackers manage to get through the chokepoints. Attackers and defenders have been designed to have a 0 s difference on being able to get to the flag when the attackers do not get slowed down at all. If the defenders manage to properly defend the chokepoints they will slow down the attackers, if they manage to slow them down by 6 minutes they win.
So far so good, technically the map is fairly geared to attackers compared to some maps and very fast paced and focused on maximizing time spent fighting compared to running. Some maps have like 40-50 seconds for the fastest route as attacker and they still get won. The difference is that those maps have more spread out points of interest, which means defenders spread out more and a good push in one direction can grant the attackers a local numerical advantage, for instance at the flag.
Specifically with my map the problem is the central location of the flag. The two side ways in, the backdoor, the tunnel, they all lead to a position very close to the flag. The defenders all spawn in the vicinity of the flag, due to my hatred for spawning very far away from the flag as a defender which makes you unable to reach a flag in peril. The flag is also very visible from almost any point of interest on the map. Which means there is barely any incentive whatsoever to spread out for defenders and camping the flag simultaneously counters every single tactic perfectly. So even though the map is technically extremely balanced compared to others, the chokepoints are too easy to defend all at once by just staying near the flag. Which makes the attackers never get a numerical advantage anywhere, so they get wrecked for six minutes.
Fixing the map requires creating locations of interest which cause defenders to spread out and stay away from the flag, which allows attackers if they push one direction properly to gain a local advantage. Also the flag location should be more concealed so the backdoor or any other form of surprise can actually be effective. My apologies, it is my first siege map and I figured that balancing the effective time to flag for both teams would be good enough, this is a learning experience. The new spawn timer does mess things up a little especially because I balanced so rigidly based on distance to flag timers, but even with high population teams it doesn't work because then the chokepoints get locked down.
