When you click on the "PDE" tag, you're redirected to a new tag "PARTIAL DIFFERENTIAL EQUATIONS". And you find all the old questions that were tagged as "PDE". Except that when you open the page of top users ( here ), all the reputation gained by members answering question tagged as pde is gone. I thought they just changed the name of the tag but this is not the case.
As Martin mentioned that's discussed at Why is the PDE tag abbreviated but not ODE or SDE?
When creating a synonym there are two options, combine it with a merge, that is change the tag on all existing questions, or keep the already tagged questions as they are.
The advantage of the latter is that it is more reversible. That is, when one cancels the synonym the status before the synonym is restored.
Apparently when one switches the direction this is treated as a synonym without merge, which I did not realize and find a bit non-intuitive. So all the pde questions stayed with pde and were, it seems, not counted.
I now did the merge and the page should update soon.