Using Outlook 2016, windows 7, hosted exchange on O365. It also happens on my Windows 10, Outlook 2016 machine.
When I click on a flag it may take a minute or two to show up; if at all. If I use ctrl key to highlight emails with flags and mark them complete nothing happens. I need to highlight them one at a time, then click on compleete for a check mark to appear.
I have created a new outlook profife two times and the issue is still there. I have compacted Otlook and issue is still there. I have changed the flags color and issues are still there.
I have noticed other people i are having simuliar if not the same issue in and outside of domain and tenent.
Any and all help is appreciated.