Downloaded 3.1 beta 2 and testing with some of our saved projects.
Context: we have been unable to move to 3.0 because of new bugs introduced. Our legacy projects and footage don't work out-of-the-box, and attempts to troubleshoot haven't gotten us to a satisfactory place.
Observed: the same sort of problems that have kept us on 2.9 still persist in 3.1 beta 2.
Working our way through a composition, one media (MJPG clip with FFMPEG renderer) renders as a solid grey color. I switch to AVF and it renders with poor performance. I switch back to FFMPEG and it renders as a solid color again, although this time the color is a light blue coming from the clip [wild guess: the difference is that when the layer had never been used it ended up with an unrelated grey color, but after running the footage through AVF to get some pixel data in the render pipeline, switching back to FFMPEG it rendered a color value from that]
Other times we have seen:
* footage that is a flashing solid color, as if every frame it was getting a single new color value from the source footage
* clips that never load, causing us to try switching decoders in an attempt to shake it up. this usually leads to the solid color problem
* a layer in our saved project was rendering the footage as solid grey, but creating a new layer the same footage rendered correctly