Did this happen when you upgraded to v7.71? Is your favorites list being kept inside mirc.ini as the [chanfolder] section as it used to be, or as the new default of a separate channels.ini file?
At least in the 7.71 installer, it has optional choices during the install, and has several checkboxes for things like the servers list, or the favorites being kept in channels.ini is listed separately. If you choose 'full', then yes all boxes including 'favorites' and 'server list' are checked. But if you choose 'upgrade', then those 2 boxes are not checked. I did a test 'upgrade' install of 7.71 on top of an existing install, and it did not reset my edited favorites.