Calendars are not added/removed in realtime when enabling / disabling a Google account's calendars via GNOME Online Accounts (GOA) in GNOME Settings
Tested with GNOME Calendar 45.x/nightly, if you toggle the "calendar" functionality on/off in a Google account within "GNOME Settings" (aka GNOME Control Center), the change in calendars available to GNOME Calendar's GUI (its popover menu, the Manage Calendars dialog, etc.) do not get reflected until a restart of the application. This can be very confusing for users.
Evolution reflects such changes in real time, so there is no technical reason why it can't be done. If it's not easily guessable per se, someone could look at how Evolution does it to do it in GNOME Calendar... so I presume a newcomer contributor could fix this without too much difficulty.
Note that the issue does NOT seem to affect NextCloud GOA calendars, only Google GOA calendars (surprisingly enough)...