Definitely not Newton rings. I can reproduce the rainbow with quite a few of my lenses. I suppose it's the shiny edge of the protective glass, you can see with your naked eye how reflective it is. It will need some kind of cover or a replacement.
This will need an optical solution, not a firmware fix. If you can do it on your own? Only BM can tell. If the camera would be out of warranty, I'd do a very careful job with Japanese model paint in matte black, it has helped me a lot with less than perfect adapters for vintage glass
BTW, this whole issue with reflections from filters behind the lens is not a new one. Red had to struggle with it from day one, when the Red One came out and made regular patterns of red dots if you were shooting into the sun – or was it branding?
The problem raised it's ugly head again with the Dragon sensor and they needed more than a year to completely fix it with a new OLPF. Now, BM is pretty new into filters behind the lens. The whole issue is much more difficult with digital sensors than with film, I can provoke such artifacts even with an Arri Alexa under extreme conditions.