Pixelmator Pro Deband feature does not work on banding caused by the well known electronic shutter lag.
The Deband feature has not been advertised as working also with ES-induced banding, so the fact that it can not handle it is not a bug. However, this limit seriously reduces the appeal of the new feature.
I have flagged the issue to Pixelmator Pro Support: currently, they are out of work, but eventually, they should get back to me with some news. Good news, hopefully.