In case anyone else runs into this:
iTunes 12.6.5.3 on El Capitan, ipad mini 5 running ios 13.3, subscribed to Apple Arcade.
I finally got around to backing up and syncing my ipad this afternoon. The backup was normal, but when I synced and it got to the app syncing part, iTunes popped up a warning:
“This computer is no longer authorized for apps that are installed on the iPad “ipad mini”. Would you like to authorize this computer for items purchased from the iTunes Store?”
“If you do not authorize this computer, 16 apps, including “Tint”, and their data will be deleted from this iPad.”
The computer is correctly authorized for my apple ID (the only one I’ve ever had) and happily downloads things from the app store and plays apple music. iTunes only named one app with no way of discovering the others, but that app was an Arcade game and there are 16 arcade games on the ipad, so it seems probable that Arcade is the problem. I’ve had no troubles syncing apps in the past (including with ios 13.3), contacts and calendars synced ok, and the backup had no errors.
I’ve looked for an update to the App Store capable iTunes version with no luck. I wouldn’t be surprised if there never is one that understands Arcade. I can live without app sync for a month or two while I finish a couple of games, but it’s annoying enough for me to cancel the arcade subscription sooner rather than later.