That sounds like the 5D banding issue that's been brought up a few times. I demonstrated that it also affects the 80D (same gen sensor) in a thread about the 5DIV recently: https://www.canonrumors.com/forum/i...d-in-long-exposure-on-5dmk4.37489/post-789615
Would be an interesting test for the 90D. If Canon has managed to fix this with an older generation sensor on the R, I would hope that the current generation simply does not have this effect...
On the Astro Ra it has to be fixed as well. Maybe that's why the R got more love than the 5DIV here?
M6II (same sensor as 90D) I guess can be a good lightweight landscape camera, but I'm not sure about astrophotography even if they fix banding. I'll wait for thorough reviews on M6II, but probably will wait for high-res full frame R. In 5DIV, banding only shows in my extreme night shots, so not a huge issue, but very annoying. The shot above was at ISO 3200 where there's no banding and I find it very bizarre. It only shows at ISOs < 3200.