4 dupes were digging a sump, and all suffocated. One of them kept working and dug out the entire sump (5x4) including tiles. She's still "alive" as I write this. She has 0% health, 0% stamina. I got a death message for her as well.
All 4 of them had a clear path back to good air, but kept working anyway. Red Alert was not in effect.
I was setting specific small areas to priority 7. In this case the ladder and the sump area where they all are now. There was some water in the reservoir, but no steps larger than 2 blocks and they would always have been able to get their heads above the water and the CO2.