Unidentified update message for iMac or iPhone 12

A message appears on my iMac that "a software update is required to connect to your iPhone" when I plug the phone into the iMac to charge it. It doesn't say whether the iMac or the iPhone needs an update. When I check the status of each device, each tells me it is up to date. The iMac connects to the iPhone and backs it up as usual. The message appears to come from Apple, but it is not in a format I have ever seen before. Is the mysterious message really from Apple?