The Vienna Convention on the Law of Treaties defines treaties as agreements between states. But the EU is not a state, it is an international organisation. So, the Withdrawal Agreement does not have the status of a treaty. The EU is not a party to the Vienna Convention, so it can hardly invoke its provisions.
https://legal.un.org/ilc/texts/instruments/english/conventions/1_1_1969.pdf
(See House of Commons Library, Could the Withdrawal Agreement be
terminated under international law? 19 March 2019.)It is legitimate to terminate a treaty if the terms of the treaty become injurious to a state: Tuilleadh
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