For what it's worth, I didn't do a reset and my save problem in 2020 went away. For the last several versions, I have imported my settings from the previous version. It's gotten much better over the years. It used to work pretty well, but not import everything over. So far in 2021 testing, it's imported everything except my double click actions.
I agree, not importing is the safest way, but it takes weeks of using the software before I get everything I customize sorted out.
It doesn't matter whether everything is set as desired.
It is annoying when not everything is imported, but nothing more.
The problem is that "impossible" behavior can occur.
I'm not talking about wrong settings that could be changed again,
but from completely impossible things (starting from a working standard version)
I used to find it good for people who don't know Acad well,
but in many years I have seen many cases and analyzed some of them in detail,
since then I would not recommend it to a beginner.
If you have structured your system well before, you need 5 minutes for the integration, actually only one minute.
Sebastian
I actually just tested this in AutoCAD 2021 and am having the same save problem. I'm not going to reset all my settings to fix it because I probably won't use 2021 anyway (doesn't add anything I'd use or fix any bugs from 2020). If I keep testing it, I'll try to pay attention to any other settings that might be changed and see if it solves the problem.