You don’t HAVE to revise – it can be argued that, at the time of the decision, the decision was correct. However, the majority of LAs would revise their decision (assuming you are now “happy” that the evidence is sufficient).
Just for clarity, the LA needs to be clear which decision it made. Was the decision:
a) that the clmt failed to provide evidence (a perfectly legitimate decision in its own right), or
b) that an adverse inference was drawn (presumably based on the failure to provide evidence).
The clmt has the right of appeal.
NB: A Tribunal cannot consider events occurring after the date of the decision, so will not (legally) be able to look at the late provision of evidence if the clmt appeals.
Regards