“Ghost” calendar inside Sync settings

Hi, I’m an Italian customer of Apple. I have in the 2-3 last years a curious issue that I haven't been able to fix it in any way on my iPhone (with EVERY iOS version, betas included).

Premise n° 1: if I go inside Settings > Calendar > Account, I see only 3 accounts: iCloud, University (based on Microsoft Exchange) and Outlook (always with Microsoft Exchange ); the ONLY account that have Calendar function enabled is iCloud.

Premise n° 2: inside Calendar app, tapping at the bottom “Calendars” shows only 2 calendars, iCloud (that I named as “Personal”) and “University” (that it is a subscription calendar of my university based on Google Calendar that it’s synced from an external URL).

Issue: inside Settings > Calendar > Account > Fetch New Data I can see a strange “another account” that I can see ONLY here, not on iPadOS, macOS, watchOS nor iCloud web interface. This account probably (?) is kind of old calendar based on Google account or something, nothing of “malicious”. But how can I remove it? I can’t find a single way to do it… I have started to thinking that this is something like a bug of Calendar app/Settings app.

Thank you in advance for your help.