Looking at the image you provided, I would put money on a reflection being your culprit. Buckeye masks are not immune to this either. I've seen it happen on folks who have them. I dealt with this when using 36mm filters as well. I worked with the guys at Agena and their first thought was to flip them as well. I started doing a little bit of digging, and soon the issue became obvious.
You can clearly see the reflection on the edge of the filter masks in this photo. Now in this case, these aren't buckeye masks, but they still reflect a tad in the same way. If you see this at all, you've likely found the reason.
Try this: Shine a flashlight down the end of your OTA so you can see all the way to the sensor. Look for anything shiny. That is going to be your problem. Feel free to post a pic in here if you do. My solution was using a 'vantablack' paint that I found on amazon and painting each filter mask individually.
The difference is huge, zero chance of light reflecting off the masks. This solved my issue.
I do have to ask though, what camera are you using because your filter edges really shouldn't be that far from the edge of the frame. It's almost like you're using full frame with 36mm filters. I'd hope not, but it 'looks' like it. Either that or your filter wheel isn't centering properly.
