For new message, when I type next to "to", very few of my contacts show up even though I have been texting them for years??? Only their emails are showing up for the majority. Randomly, a few contacts will have a mobile number I can pick to message. Some contacts don't show up at all even though they are marked as "favorites"--all very bizarre. Sending a group text is next to impossible because none of my contacts mobile numbers are showing up.
Also would help to know if this specific device is the same one that's been in use for "years", and whether you've been icloud synching your contacts or not.
I think there’s a simple solution - create an actual Contact for them.