Question:

Why do some Call Key Message records not have a Mobile ID value?

Answer:

This is working as designed.

When the related Call record has less than 25 Key Messages associated with it, the related Call Key Message records won't get a Mobile ID value.

When a Call record has at least 25 Key Messages associated with it, then the Mobile ID field gets populated on the related Call Key Message records.

Related Documentation:

CRM Help Documentation: Using Key Messages on the Call Report