While this may indeed be a sensor problem, perhaps you can check if this is a case of a light leak.
I notice that the ZWO T2-1.25" adapter plate that I bought (perhaps within the past 6 months; from Agena) has the same part number (ZWO T2-1.25-3) as the one shown at ZWO's web site, but it does not have the 4 holes. Instead, two of the "holes" are drilled only partially from one side of the plate and does not go all the way through, while the other two holes are partially drilled from the opposite side of the plate. As a result, you can still remove the plate with a lens spanner, but light does not pass all the way through.
I wonder if the design of this plate was quietly changed because of potential light leak problem.
If you have the T2-1.25" plate that has the four thru holes, perhaps you can try using black electrical tape to cover up the holes, to see if the flats looks any different. Black tape can still pass IR, so it may not completely cure the problem, but if the flats are different, then it is an indication of the culprit.
The narrow band filters block out a lot of broadband light. The red component of a Bayer camera probably passes around 150nm of passband, and even a crummy 15nm HA filter will only let through 10% of that, so any light leak is more obvious than when not using a narrowband filter. It would be much more obvious with a good 5nm filter of course. This is why I am always paranoid (and perhaps overly sensitive to the issue) of light leaks.
Depending on how the T2 adapter of an ASI533 is threaded (I don't own a ASI533 to know), perhaps you can use some tape to cover every possbility also. Perhaps also tape up the interface between the T2 barrel and the camera chamber.
Finally, you can probably test the light leak conjecture by using a 2" duo narrowband filter ahead of the 1.25" inch one, but I suspect that you only have the 1.25" one, otherwise you would have chosen to use a 2" filter with the camera.
Again, this could be something completely different from a light leak, but if you are out of ideas, you might want to spend a little time investigating.
Good luck,
Chen
