They are attacking once for subduing damage and then switching to lethal damage, which means the foe is automatically kept alive but unconscious. The ratio of subduing damage to lethal damage can be low, such as 4 out of 40 HP. That doesn't strike me as the intent of the rules.If a character is reduced to zero hit points who has taken at least some subduing damage, the character becomes unconscious rather than dying. (Any further subduing damage is then considered killing damage, allowing the possibility that someone might be beaten to death.)
I'm inclined to make a house rule to change "at least some subduing damage" to be "subduing damage greater than half maximum hit points". Alternatively, I'm considering requiring the final blow be subduing damage, else it's a kill.
Nine years ago, there was a brief conversation on the forums about a "rule lawyer loop-hole". Fortunately, my players have not yet schemed to use subduing damage on themselves as a protection against death.
Is this a problem to be fixed, or should I just let them go on trying to knock everyone out?