Can not send SMS to one contact
Apparently this was fixed with one of the recent iOS updates.

Currently I have iOS 13.3 and this problem is gone.

Edit: "fixed" is an overstatement. I can now send a message that arrives by manually selecting the correct phone number. If I write another message in the same thread, it is again sent to the wrong number. The RECENT label, if inspected, is again on the same wrong number, as before the "fix".