[PROBLEM] Some settings not saved correctly

[the_bertrum]

As a last resort, I swapped the PI3B+ with a P4. Problem hasn't occured anymore since more than a week.

No idea why the behavior on a 3 is different than on a 4, but it seems this fixed the issue.

May not be related but....

I've just spent several weeks tracking down an issue on a Pi3A+ where it would randomly loose its wifi connection and have to acquire new one, playing havoc with streams and mDNS and so on.  I even returned one unit to the shop thinking it must be faulty.
Turns out there was a "smart" feature on my router that attempted to push connected devices on to the best frequency which all my other RPi devices where OK with (a zero and two 4B+), but the 3 didn't like at all.  I've told the router to stop being so clever, and now the 3 works flawlessly.

Frankly, since the only devices I have that would benefit from the "smart" router are the family phones, and they are quite capable of switching to a better frequency by themselves, I'm not loosing anything here

Anyway, the point I'm making is that I have experience of the Pi3 being sensitive to oddness on the network, perhaps that's what's going on for you as well?