My recent calls list doesn't contain names, only phone numbers, since upgrading to iOS 12

First try restarting your phone. ( actually happened to me, that fixed it.)

( but- did you recently change the SIM card? Maybe there s pending carrier update.

Update your carrier settings on your iPhone or iPad - Apple Support

after you check that, if issue still is there , check that your Language + Region setting is set correctly. Go to Settings > General > Language & Region.)

That means that the entries are missing from your contacts, which is where the Recents list gets the names.

Just thought if something.

settings - iCloud-turn contacts off, wait a few seconds, turn back in. Restart your phone.