Contacts.app misbehaving in Sequoia on M1 iMac

iMac 24” M1 2021, macOS Sequoia 15.1.1, Contacts v. 14.0 purchased from the Apple Store Refurbished January 18, 2024 Since updating my partner’s iMac a few weeks ago, Contacts is misbehaving: New lists can be created within Contacts, and populated by dragging individual contacts from the “All Contacts” list and dropping them onto the new list name. On this machine, however, this usually works only for adding three or four names. After that, dragging a contact onto the new list name has no effect at all on the new list name (that is, the list name doesn’t change color or highlight), and the new list, when opened, does not include the unsuccessfully-just-dragged Contacts entry. What I’ve tried: • Quit and relaunched Contacts. Temporary improvement only • Restarted the computer. Temporary improvement only • booted into Safe Mode. Misbehavior continues after reboot • run Apple Diagnostics to make sure the hardware is OK. It’s OK. • reloaded macOS using local Recovery. Temporary improvement only After doing any one (or more) of these, Contacts behaves correctly for another small number of drag-and-drops, and then fails again in the same manner. In all other respects, Contacts behaves correctly. Multiple complains now appear in the Apple Community about Sequoia contacts not synching. The problem on my machine, however, doesn’t seem to be related to iCloud synch, as the new list including the contacts I’ve been able to add (as well as the entire “All Contacts” list) appears correctly both on the synched iPhone and also on the iCloud web page. The machine is still under AppleCare+, but I’d like to find and rectify a cause, rather than spending time on the phone with Apple Support, or physically returning the computer to Apple or to an Apple Store. Any ideas?