Cannot add contacts to Screen Time > Communication Limits for my child's Apple Watch

Within the Watch app on your iPhone, navigate to Screen Time > Communication Limits.

Choose Contacts. No Contacts appear if this is your first time. Contacts count will also be 0.

Add a Contact e.g. Mom

You are now back at Screen Time > Communication Limits

Figure 1: After re-attempting 6 times, the Contacts count shows 6.

Figure 2: Adding a new contact still displays an empty list as if the previous 6 attempts never happened.

Contacts count is 1.

Add another Contact e.g. Dad

** No contacts appear as if the previous contact didn't save

Go back to Screen Time > Communication Limits

Contacts count still says 1

Restart both devices and re-attempt.

Same outcomes as above.

This is reproducible on iOS 17.3.1 and WatchOS 9.6.3.