I used the Momentum 4 with FW 2.13.28 two more weeks with ANC on and off. I'm now sure FW 2.13.28 severely broke the ANC. While I'm typing this, vacuum cleaning is going on and the ANC (slider 100% to left) cancels out nothing!

I notice exactly the same behavior since last week. I'm also on FW 2.13.28, but on HW version 3.0. In my case it is not fixed by enabling and disabling adaptive ANC. My best guess is that this bug was introduced with FW 2.13.28.