Interesting. So I separated deletion out into a first step. And afterwards I check the folder in Finder: empty.
Then I waited. And check the folder in DEVONthink: Still full with the documents! (But when I preview a document it now says the document is not available.)
I waited longer. Over night. And the folder appears in DEVONthink 3 still with the documents. Only in the Finder it is still empty.
No wonder the Smart rule doesn’t kick in!
Time to move this over to support?