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CRA policy instructs debt collectors to delete their reported collection if they sell the debt to another.
The purpose for that CRA policy is that once sold, the new owner can report their own collection, which can then result in two collections simultaneously included in your credit report for the same debt. Deletion of the first collection is intended to prevent that situation.
However, since they did not delete with all three CRAs, it is possible that they simply decided to simplify their reporting by deciding to only continue reporting and updating with one CRA. They are permitted to report with only one CRA, and there is no basis for disputing the continued reporting with only one CRA if they still own the debt.
If in doubt as to whether they still own the debt, you can always give them a call.