Replying to myself. Talked with Apple Support and came to the conclusion works as designed ;-) Anyway, here's what we found:
In reference to Case #************ on 9/13/2023
It seems when you start a text message by entering the first few letters of a contact name it drops down a few possibilities. If the contact has multiple numbers and you just click on the entry shown it selects the lowest numbered phone number regardless of positioning or naming in the contact entry.
If you click the ">" of the entry and select entry you want, i.e. put a check mark next to a different number, it sends to that number but does NOT remember that selection if that entry is deleted from the history. If it is in history, it will remember the selection, as if it's adding to the existing conversation.
So... two 'problems': can't control the 'default' selection (it always picks the lowest numerical number, and also, it doesn't remember the 'checked' number selection if that number is not in the history.
I sent feedback so maybe someday...
[Edited by Moderator]