I use acalendar+ but the problem is there with Xiaomi calendar as well. If acalendar finds the account it says there's an internal sync error. Under accounts and sync I can't enable acalendar sync. It shows Calendar but won't sync. Google Calendar does sync. It also says "sync is currently experiencing problems. It will be back shortly." - this has been going on for a few days.
Both Google Calendar and acalendar show a strange numbered account which could well be the numerical version of my google account. acalendar calls this account calendar_displayname. I can't find this account in accounts and sync to remove. Reinstalling the google account itself and the acalendar app don't help.
Uninstalling Google Calendar and reinstalling allows Calendar to sync, but still shows the numbered account in Google Calendar. When I open the Google Calendar after reinstalling it says "Sync is off for the numbered ccount. Changes made to this calendar won't be backed up". I can dismiss or enable sync. So I enable it.
Deleting / adding events in non google calendars seems to work on storage but won't sync, so doesn't show up in Google Calendar.