I see that starting from 7.4 we have an option to upgrade multiple ADOMs
at once but they have to be on same version.
https://docs.fortinet.com/document/fortimanager/7.4.0/new-features/107391/fortimanager-can-upgrade-multiple-adoms-same-version-at-th...
Well, there was an earlier bug#1119299 about that but it was fixed in
7.4.7 and 7.6.3, but you said you are running 7.4.8... better to open a
ticket and check with support.