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?