To be honest I would call it inefficiency rather than a tactic. There are two things happening with a direct debit, the person owed debits your account for the money (the opposite to a standing order where you control the sending of money), the person owed would not know always know a direct debit mandate had been cancelled, now I know in your case it's the same bank but just for explanation purposes bear with me! So the person owed the money still goes ahead and debits your account, then the current account holding bank should reject that debit as the mandate has been cancelled so often you can find that the debit goes through as the owed bank collects but is then claimed back by the current account bank when the system realises there is no valid permission in place to pay it.
In other words sometimes these things right themselves the following day when the system spots the error, in your case while the bank cancelled the direct debit mandate on your current account they obviously did not do the cancellation on the mortgage side whether deliberately or not who knows!