1. What happened
For 'Chronos - Time Control' A$ package, when switching from New Runtime to Stable, after Editor restart and project reload, I'm getting lots of exceptions, like "TypeLoadException: Could not load type 'System.Runtime.CompilerServices.ExtensionAttribute' from assembly 'Assembly-CSharp'."

Also looks like something gets broken within Editor itself - I've tried opening 'Player Settings' and those looked as if in Debug Inspector mode, although being in Normal.

2. How we can reproduce it using the example you attached
The project is in a state after-the-issue-had-occurred.
To get a new one:
- New project
- Import 'Chronos - Time Control' A$ package
- Switch the runtime to the new one (+ restart)
- Switch the runtime to the old one (+ restart)
- See Console for errors