Sadly this does happen fairly often, sorry this happened to you 🙁 . The thought is that they smell different when coming back from the vet. In most cases the buns can be rebonded , sometimes quickly. Best bet is to keep them separate as the vet suggested, and go back to pre-bonding (side swaps). Then do bonding sessions in neutral space as you would if they had never been bonded. In many cases they just need a few days of sessions to “snap” back to being bonded.
Next time you need to take Ollie for a trim, Cinnimon should go with him. If that’s not possible, bring her in the carrier to and from the vet, and rub a towel over both buns to swap scents. This usually helps them “remember” each other.
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The answers provided in this discussion are for general guideline purposes only. The information is not intended to diagnose or treat your pet. Seek the advice of your veterinarian or a qualified behaviorist.