I don’t think that’s the issue.
There is one IPad.
iPhone A has only my personal-life contacts. iPhone A is not linked in any way to the IPad—no contact sharing, nothing.
iPhone B has my BUSINESS contacts. Only IIPhone B is linked to my iPad. I can see business contact “Susan” on the contact lists of IPhone B and the IPad.
If I use iPhone B to text, say 7 business contacts (one is Susan) during the day, I can come home that and see on my IPad all texts with all those people—EXCEPT the text exchanges with Susan.
Why not hers?
if I use iPhone B during the day to text, say 7 different people, I can later see in my IPad