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

Recently, the Messages app on my iPhone 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*, Messages displays **+1**-*nnn-nnn-nnnn* which is not recognized as a Contact. Or the opposite: the Contact is **+1**-*nnn-nnn-nnnn* and Messages shows *nnn-nnn-nnnn* which is unrecognized. Any idea why this behavior suddenly began? The problem happens only on one IOS device (my phone) and not on my iPad, even though they sync Contacts through iCloud. Re-syncing contacts did not fix the problem. Restarting the phone did not fix the problem. As a hack, I added the new number to the existing contact. So a given contact now has both versions of the phone number, so Messages recognizes it and displays the person's name. This seems ridiculous when I live in the US with country code 1. Any suggestions appreciated. Thanks!