First my device background. I have a Samsung Galaxy S8+ on Verizon with the latest security patches. Android 8.0 firmware version G955USQS5CSB1. I'm not sure if that matters. I am using the stock + patched gmail app on the phone.
I am an office 365 user, although I also have a couple other accounts that are on-premise Exchange.
Whenever I receive a calendar meeting invite, if I accept it on my phone, the calendar meeting gets updated fine, the message goes to deleted items - fine, but I then have an unread count of 1. Every meeting I accept the count goes up by one. It never clears. The only way I have found to clear that is to remove the entire account and then re-add.
If I accept the message in Outlook on my PC I do not have this issue.
I haven't tried the outlook app on my phone - I don't like it that much.
I also have an iPad mini, latest patch version, and I don't have this problem there.
This isn't a particularly new problem, but I accidentally accepted an appointment this weekend and I'd really be happy if there was another solution before I remove all my email and resync again, and more importantly that I might someday be able to accept meeting appointments on my phone.