That's why they have to keep monitoring and keep digging. At the moment they can see the numbers, but they can't be sure of the trigger and whether it really is the vaccine. What they also want to know is whether they're looking at the normal background rate of this condition, which varies between age groups and across sexes.
If a link to the vaccine is confirmed, whether that's as the sole trigger to the condition or as the "last straw" for someone who was already on the border of it - they have to go deeper again on the mechanism.
Initial suggestions are that if it's happening to AZ but not to the mRNA vaccines then the chances are that it's the adenovirus that does it. If that's the case they may see the same problems on mass rollout of Sputnik and J&J. Which, for most of the world who were relying on these cheaper, fridge stable vaccines is a big deal. That's why the numbers and risk factors have to be understood. Phasing out is a reasonable decision, but slowing the vaccination program in countries with high case rates and particularly amongst high risk groups or scaring people away from vaccination will kill more, and it's important to get the balance right.