Birthday Added to Contacts is Disappearing

When I add a birthday to a contact, within a few seconds of the addition, the birthday disappears. It is no longer in the contact, and it is no longer in the calendar. I have seen it actively disappearing from the calendar a few seconds after adding it to the contact.

For reference, all my contacts exist within the iOS Contacts application, not Google.

Model: iPhone 11

iOS Version: 17.5.1

Steps to Reproduce:

1. Open the Phone app
2. Tap the 'Contacts' icon on the bottom navigation bar
3. Choose a contact without a birthday previously entered
4. Tap 'Edit' and add a birthday to the contact
5. Tap 'Done'
6. Close the Phone app and open the iOS Calendar app
7. Navigate to the month and date of the birthday that you just added
8. Observe that there is no item for that day related to the contact's birthday
9. Close the iOS Calendar app and open the Phone app
10. Search the name of the contact whose birthday you entered
11. Tap on their name
12. Scroll down to see if the birthday has been saved to the contact
13. Observe that the birthday is no longer listed in the contact's details