does this keep happening with one particular event or are you having a similar issue with all events?
What version of eM Client are you using?
I have the same issue. Every time I start the Computer, or was in Fullscreen, there popping up the reminder. That are Calender reminder, weeks ago. Only to erase the date, the reminder is gone. 8.0.3385 (83a873c) this is the Version.
I like your eM Client and I think it is good to pay for that, but this problem is very bad. It is a Calender from my Nextcloud.
I have this same issue with Outlook/Exchange calendars and have had it for many versions. V9 still has the same problem. I’ll just suddenly get the same reminders over and over and over and over. Even ones I’ve previously dismissed will suddenly appear multiple times at the same time in my notifications. It is pretty maddening since I can get tens of them all at once and have to dismiss them all.