iPhone calendar syncing problem

This problem involves my wife’s iPhone 13. She and I both have iPhone 13’s, and we both have 2024 iPad Air tablets. All are up to date with all updates. All are configured to use Google calendar using the native i(pad)OS calendar app. The strange behavior relates to my wife’s iPhone only.

Here is a synopsis of the behavior.

1. A calendar entry created or edited on any of the other three devices will appear in the calendar app on ALL FOUR devices.
2. A calendar event created on one of the three other devices and synced to my wife’s iPhone can be edited on her iPhone, and it will properly sync to all other devices.
3. A calendar event created on my wife’s iPhone WILL NOT SYNC to any of the other three devices.
4. As near as I can tell, both my iPhone and hers are set up the same way for calendars. The only difference between her iPhone and iPad and mine is that she syncs Contacts to Google, whereas I sync to iCloud.
5. Her iPhone did properly sync calendar events in the past; this is a relatively new problem. Not sure when it started; perhaps a couple of months ago.

Does anyone have suggested fixes? This is quite frustrating.