Just curious, did you try to resize the sub-frames during processing?

I had similar issue, when I tried it with one of my image sessions (Shot with wrong driver in NINA). But, the artifacts were horizontal rather than vertical.

I was able to plate solve your XIIF in Siril (Local GAIA database).

No success with Siril BGE or GraXpert BGE/Denoising or Seti Astro ABGE with your XISF file.

Easiest way to tell if it's due to the flats is to stack without the flats and see if the issue is still there.

How long is the exposure for your flats? If you use something like a laptop/tablet screen, and the exposure is too short, you can end up capturing the flicker of the screen as it refreshes. I find that an exposure of at least 200ms works well and seems to avoid this issue. You may need to add more layers of fabric so you don't overexpose with the longer exposure.

You didn’t say what kind of camera. This looks like a calibration issue to me. Bad flats/bias/darks could be any of them….

Try running subframe selector(there are tutorials on YouTube) cull 50% of your frames and stack without calibration frames. Re-introduce each calibration frame type one at a time.

If you’re shooting a DSLR, make sure you’re doing your bias and dark frames every session.

How many calibration frames were you taking?

The big question is whether those effects are in the original data or if they have been introduced by the processing. I usually perform ABE with function degree 1 to assess the original data because this simply subtracts a linear gradient without introducing potential anomalies.

You can also check for potential anomalies in your master flat by dividing one colour channel by another (e.g. $T[0]/$T[1]/10 or $T[2]/$T[1]/10 in PixelMath) and then applying STF

For me ImageSolver worked fine on your xisf file to give an astrometric solution.

Check ur subs, this could be few bad/cloudy subs that's messing with your stack. I wouldn't rule out flats thou.

I ran 2 times ABE to it and it stopped being so weird when nuking it, but there is some shenanigans going on.

Also, it has better looking* backround in the top right corner, that could be a tell that few bad subs have slipped in ur stack and their ruining the overall picture.

Try blinking or subframeselector for the raw files and stack only those that pass inspection and see if that problem goes away.

Try MultiScaleGradientCorrection or GraXpert and see if you get the same.

Sometimes DBE can be the reason for this. Have you tried with ABE? I find sometimes this gives me a better result. There could also be stacking artifacts along the edges. It's best to crop these out before using DBE.

I'd recommend attempting undiffused skyflats (no t-shirt or other material) taken about 10-20 degrees offset from zenith during a setting or rising sun on a clear day.

Do you have a link to your master flat?