Did you have the same documents as you had with 40 shards?
As far as I know, no.
It looks like the documents that match your query have no PDate value, thus elasticsearch internally uses the Long.MIN_VALUE to sort them because missing is set to _last and order to desc. Using Long.MIN_VALUE helps to "push" documents with no PDate value to the end of the list but in your case only documents without PDate match your search query.
I see that also the _missing mode value gets ignored. I retrieve the docs with the PDate field in the source, and then I see its value(s) present, and even fields with missing values get shown in the top. The sorting value is for each and every hit this: