I have the weirdest bug. If I edit anything in the 'Script Execution Order' list in Project Settings, and click 'Apply', it just reverts back to how it was before immediately. This includes any type of change: removal of entries, adding entries, or changing the number of the entry. Everything just plops right back to the previous state on clicking Apply - no error messages whatsoever.
Visually, it's exactly the same result as clicking the 'Revert' button instead of 'Apply' - except that the editor actually starts doing its refresh thing afterwards (I thought for a moment maybe the Apply button was broken, but this kinda rules that out).
After hours of trying stuff I figured out that it works if I remove the 'Unity UI' package, so I guess the problem may have to do something with that. However, another project that includes this package does not have the same problem.. so it's still project-related, somehow. I tried uninstalling and reinstalling all packages 10 times, no luck..