Had someone ring me complaining they had a wrong email address in autocomplete. I told her start typing it, highlight the (in)correct entry and hit del.  Didnt work.  So the next suggestion was delete the autocorrect cache.  Still didnt work! I thought why not see if it does the same on the OWA and yup, the incorrect entry is in there too! 

I then went on to try doing the same on my Outlook and its doing the same!  There doesnt seem to be any way to clear an entry from autocomplete anymore!

Both are running Microsoft 365 with the latest Classic Outlook, both have the same issue on the webmail.  One computer is running on an ARM64 surface pro so I thought maybe that was an issue but doesnt seem to matter.  

Instructions when searching for the same issue say there should be an X to delete an entry, or use the del key, or delete all the autocomplete. There was definitely no contact in the address books.