The normal reason for unexpected deletion of a collection is that the debt collector's collection authority has been terminated.
If the debt collector has assigned collection authority, then termination can be done by either the current owner who assigned the authority, or the assigned debt collector.
If the debt collector owned the debt, their collection authority is automatically terminated if they sell the debt.
Either was, if the debt remains unpaid, then CRA policy instructs the debt collector to delete their reported collection.
If a new debt collector becomes involved, they can report their own collection.
There may be other reasons why a debt collector has voluntarily deleted. If unsure, you can give them a call.
If they no longer have collection authority, they cannot negotiate any payment of the debt.
