There is some asymmetry going on so I think there is a little bit of a tilt component, although could be caused by a miscollimated cell. Whetver is going on it's really bad. Even stars in the center have a slight elongation to them.
When you say your last shot looked "pretty clean" is that code for... not good but not too bad? For your last shot was it the same camera, or has anything changed in the imaging train?
You can easily do a test to see if your spacing is playing a role.
1) Achieve perfect focus on a center star. Note the focuser position.
2) Now focus on a corner star. It will probably "round out" and look much better with a little focus adjustment. Note whether you needed to move the focuser IN or OUT to get better focus.
3) If you needed to move the focuser OUT to get better stars in the corner, you need to REDUCE your spacing between the flattener and your sensor.
If you needed to move the focuser IN to get better stars in the corner, you need to INCREASE your spacing between the flattener and your sensor.
Based on what I am seeing, there is a problem with spacing. BEFORE I would start playing with adjusting a lens cell, perform this test to see if you can get it better with spacing. I'm not saying this is the only problem you have, but I do think it is a problem that you have.
I would first dial in the spacing, second look for a source of tilt and third if you have been able to dial in the previous two... start suspecting the flattener and/or the scope.
These issues can be maddening! Good luck!