From what I read, the door plug on the MAX-9 is the same or very similar design as to the one on 900ER. Delta was the last operator to take in a bunch of 900ER in 2019 just before the pandemic. Whether a design issue or an assembly quality control issue, I think the FAA (or even Delta itself) should ground the 900ER as well for inspection. On medium haul, it's hard to stay away from 900ER... Safety needs to be #1.