Why does Messages treat US phone numbers with and without +1 as different contacts?

Recently, the Messages app on my iPhone and iPad stopped recognizing many of my Contacts — it displays their phone number instead of their name. On closer examination, I noticed something odd. When a Contact has a US phone number like nnn-nnn-nnnn, and I receive a message from the equivalent number +1-nnn-nnn-nnnn, the second one is not recognized as a Contact. Or the opposite: the Contact is +1-nnn-nnn-nnnn and Messages doesn't recognize nnn-nnn-nnnn as a Contact.

Any idea why this behavior suddenly began? FYI, my default country is properly set to the United States in Settings->General->Language & Region->Region.

As a hack, I added the new number to the existing contact. So a given contact now has both versions of the phone number (with and without the +1), so Messages recognizes it as a Contact and displays the person's name. This seems ridiculous when I live in the US with country code 1.

Any suggestions appreciated. Thanks!

[Re-Titled by Moderator]