Considering the Windrush scandal the UK doesn't exactly have the best track record with dealing with immigration issues. The authorities might - either conveni

Considering the Windrush scandal the UK doesn't exactly have the best track record with dealing with immigration issues. The authorities might - either conveniently or due to incompetence - lose relevant documents in the future, at which point you might not be able to prove anymore that you've already had a leave to stay in the UK before 31 October 2019. This also might be used as leverage in upcoming negotiations with the EU.