It seems like in some cases, calendar permissions are not properly set on the new OS. Normally it should ask again when the app updates but if not, you may check below options.
(macOS)
System Settings - Security & Privacy - Calendars: Check permissions
(iOS)
GoodTask's Settings - Lists & Calendars - Calendars: Check if it asks for permissions again
Here is a hint / tip that might help. In System Settings -> Security & Privacy -> Calendars I DID have GoodTask enabled but for some reason it was ADD ONLY ACCESS which did not share the calendar (see below).