This is an old known bug, but I’m mentioning it again because it still hasn’t been fixed even after the last update. When you kill an enemy with a blowgun, if it dies from the poison the image freezes on the screen blocking other enemies that might be behind it. (This doesn’t seem to happen if the enemy is killed outright by the attack.)
My “solution” is to poison the enemies that I can reach in the back first and then try to kill the front row enemies outright with multiple hits (trying to not let them die of poison). That way, the frozen enemies are in the back out of the way; the front enemies will vanish on death as they should; and any hidden enemies in the middle will with luck be revealed. However, this doesn’t always work out, especially if you get hit with a stun and don’t have a chance to kill off a front row enemy before it dies of poison and blocks the screen. And of course in a battle with multiple people fighting, you can’t control your teammates’ fight strategy.