Did you stabilize 18.04 over a couple days to make sure you had caught all the reconfig required? It should be no different than 18.04 > 20.04 with that step I would think. I am currently at this phase ;).
Not always possible, of course. It's also understandable that you can't just jump through 4 years of updates in one go without some sort of work to catch up to current configs involved. I had to change 2 lines of config from 16.04 to 18.04 (due to my own tweaks to defaults), so it's been pretty pain free so far. And the few days as a precaution are on me, not Ubuntu. I should have updated years ago.
And it wasn't my personal machine so it would've been unreasonable for me to do so.