What is special about the MDPI special issues? This question is related to this question, but specifically regards the special issues by MDPI. They seem to have a quite high number of special issues for every journal per year and as far as I understand, they are only different from the regular publications by having a dedicated topic and an associated editor.

One thing that confuses me is that papers are already published before the deadlines (this special issue with the deadline in October already has 4 papers published).

Is there anything else that is special about these special issues?
There really isn't a fundamental difference between their special issues and other publishers' special issues—it's just a "themed" issue.

As for publishing papers before the deadline, that's a relative feature of digital publishing—there's no need to wait for a deadline if a paper is ready to go earlier. The collection can be "built" as papers are ready, rather than presenting everything as a single fait accompli.