Charge 4 won't pair with THIS phone but will pair with THAT phone

[bcrawfo2]

My daughter has a hand-me-down (from her brother) Charge 4.   Both kids have iPhone 6s+.   I cleared user data and deleted the bluetooth connection as well as removed it from any Fitbit apps.    I go and try to pair the Fitbit to my daughter's phone and it fails...never finds it.   I go over to my son's phone and it pairs right away.   Repeat the process (delete, remove, clear data, etc) and go back to daughter's phone....no dice.

Data

Daughter's phone (phone A)

6s+

IOS 15.2.1

Fitbit 3.54(1098)

Son's phone (phone B)

6s+

IOS 15.1

Fitbit 3.54(1098)

When struggling thru this earlier, my son mentioned "I kind of remember when had this problem when we got the Fitbit.   It wouldn't pair with my phone, so you swapped phones".    That is....it didn't pair with phone A, but would pair with phone B.   I went to the trouble to backup both phones.....restore phone A backup on phone B and restored phone B on phone A.    Obviously a less than I deal solution.   Hopefully someone has some ideas.   

[bcrawfo2]

I just spent 26 minutes with a very helpful support person from Fitbit.    We concluded the call with "that's weird, it should work".   I questioned what this meant.   He had no ideas and there was no option for escalation.   It appears this would turn into a finger pointing game between Fitbit and Apple.