EDIT: Round 22**** not 27

Alright so round 22 is the official run killer. On 3 different runs now, I have found that if I spawn kill the enemies by shooting them as soon as they appear, there is a chance the game will softlock after killing about 18/30 enemies. Once the game softlocks, no more enemies will spawn and the round will never end. This can essentially kill a run because this only happens right in the middle of round 22. The only possible way to save the run is to SD and restart round 22 from the beginning, which will cost about 2 minutes. But then again you will have the chance of the game softlocking again if you kill the enemies too fast. Interestingly, this does not happen on rounds 23, 24, or 25 for Stage 5. I've spawn killed the enemies as soon as they spawn on those rounds and the game has never softlocked. Only round 22...

I've found in the Uncharted games, in particular in Uncharted 3 that there's usually one or two enemies in some sections that will cause the game to softlock if they're killed too fast. It may be worth you or someone to test who of them enemies killed is causing the softlock as I don't think it's all of them. It could very well be the last enemy in the pack that spawns. It's possible that when the last enemy spawns in that pack the game hasn't been given enough time to recognise that not only are they the last but that they've spawned.

Maybe try with the last enemy in that pack, let him spawn or be alive just a little longer than the rest and see if that fixes the issue, if not try and experiment with the rest.

Okay, I think I understand it a little bit better now. It kinda makes sense that killing the enemies too soon or too closely to their spawns could cause the game to stop registering and then softlock. I think I've found that the spawn rates and also the number of enemies that spawn, goes down right in the middle of round 22. Once you reach about 15/30 kills for the round, only 1 enemy starts spawning instead of the usual group/cluster and if you continue killing that 1 enemy, immediately as he spawns, then after about 7 or 8 kills the game has a chance to softlock. So I think you're right, I just need to give the enemies more time to spawn in that one specific section