I think this may help as I was running into the same issue after switching to a new Mac.
In the new outlook, I removed the account, re-added but this time selected the big blue button below to synch with MS cloud. I don't recall if I did this the first time when the pin function wasn't working. But after doing this, the pin functionality is back. If this doesn't work, just try few times....I really didn't do anything special. settings on my old and new MBP were identical.