I set up events in the calendar and set the tags, and after a while they are gone. When I close the eM Client on my pc/laptop and open it up again, they are back.
I am having exactly the same problem. Calendar entries mysteriously vanish. They return when I close and reopen the application. I haven’t been able to find a pattern around when or why this happens. Would be keen to know if many others are seeing this behaviour.
I’m having the same problem with new calendar events not registering visibly, but closing the application and reopening the application reveals the new event. Version 10 is a disaster. Too bad the developers couldn’t be bothered to do sufficient testing.
This has been noted before and the developers have fixed this bug in the next version update.
Exactly the same problem, makes v10 unusable 
Right - that problem sucks and needs to be fixed immediately. But there is a trick to get along right now - just click on the name of one of the missing calenders and suddenly the missing calender entries show up.
Could be this calendar issue has now been fixed in the latest release V10.0.3351.
If you update, close eM Client before updating.
I set up events in the calendar and set the tags, and after a while they are gone. When I close the eM Client on my pc/laptop and open it up again, they are back.
There is a new V10.0.3447 update via the release history page which mentions a fix for events disappearing, so suggest to update to this version and see if that fixes that issue for you.
Before updating close eM Client before updating and make a manual backup if you don’t have any backups already via “Menu / Backup” incase you need to restore for any reason.