I did further tests by trying to change the slider to see what happens with the various synchronization periods: for my test account (which is a customer of mine who reported the problem to me...) it seems to work up to six months, but if i put it to more than outlook sync stops working.
If I roll it back to six months (or less) then it starts working again, but of course I only see the emails from the synchronized period.
The worst part is that this forces me to only see the last six months in all other folders as well, not just the INBOX...
In fact, if I set the synchronization to six months, Outlook makes everything that is older disappear from any folder, even if it had already synchronized it with older items before.
So, while this may be close to a workaround, the downside is that it causes other problems that the customer may not accept…
NOTE: of course everything remains on the server, so from webmail I can still see items older than six months in any folder