iPhone backup keeps changing to "iCloud".

Since upgrading to iOS 13 and getting a new iPhone, the "Backup" option in iTunes keeps changing back to "iCloud" from "This Computer".

I want my iPhone to back up to my computer. iTunes seems to want to back it up to iCloud. Settings in the phone show that iCloud Backup is off.

From the iPhone screen in iTunes, I click "This Computer" and click "Back up now". All's well - the phone backs up to the computer. However, when I plug the phone in next time, the setting is back on "iCloud".

This means that unless I do it manually, my phone isn't getting backed up: The phone thinks it should be backed up by the computer, but the computer thinks that the phone should be backup up via iCloud.

Essentially the opposite problem to this one: https://discussions.apple.com/thread/250686589

Has anyone else seen this?

(iTunes 12.9.5.5, macOS 10.14.6, iOS 13.1.2)