I have an iPhone SE with iOS 13.2
Recently I noticed that no SMS messages arrive to one of my contacts (using an Android phone).
The contact has multiple phone numbers and I suspect the SMS is sent to a wrong one (not currently in use).
I think this started happening with the update to iOS 13.
No matter what I do, the messages don't arrive.
I had a similar problem before, because the Messages app does not show the used phone number, but only the contact name, so I was sending messages in thread A (the right contact, but wrong number) instead of thread B (right contact, right number).
But with iOS 13 it seems all threads of a contact are merged and I'm stuck with it.
If I try to start a new thread, select the contact and the correct number, I get transferred to the existing thread. Then if I type and send a message, it does not arrive.
Interesting detail: when I do the above (new message), when selecting the contact, by default the wrong phone number is selected. Apparently this number is used, no matter what I do.
One more detail: if I try to see which number of a contact was used, using his "simple" procedure: open thread, tap contact name on the top, tap "info", tap the ">" sign at the far right, then the "RECENT" mark is on the phone number that I last selected for sending SMS. But this seems just to be a GUI thing, as the message never arrives.
I don't have problems sending SMS message to other contacts (most of them have a single phone number).
I also have no problems receiving messages from this contact. They appear in the mentioned existing thread. I just can not answer.