Using the latest iOS eM Client mobile app V10.5.430 with OS26.6.1 , If you setup a eg Google account calendar event in with a reminder of “At Start” and then open Apple Calendar it syncs & shows the same alert reminder start time on the event which is good as I use both.
However Apple calendar will also set a second alert of “Time to leave” underneath it which if you change that to “None / No reminder” it will start creating a duplicate event back in eM Client mobile app.
Now I know the latest eM Client 11 Beta now has multiple calendar reminders so maybe this will be ok when the new eM Client iOS mobile app is released later with multiple event reminders, but for now don’t change that second alert field in Apple Calendar app. Support might comment on this.