Catalina upgrade has caused Calendars in Internet Accounts to stop syncing

I have a secondary iCloud ID which is used only because I want to sync Calendars and Contacts with my partner's iMac (Mojave), my old MacBook (El Capitan) and our iPhones (iOS 12.4.8).

A new event added to my iMac (Catalina) does not sync to any of these other devices, nor to this secondary iCloud account when I sign into it online. Contacts syncing between all devices is fine.

A new event added to my iPhone does, though, sync to all our other devices including to my iMac (Catalina),

Have checked that all devices have their boxes ticked correctly.