Have you tried embedding the metadata before exporting the files? That is, the .opf file in the ePub should have the series information embedded. I am not aware that MoonReader+ or any other ePub viewer will use the external calibre .opf file.
Also note that if you are exporting an ePub3 file, it will use the ePub3 standards series information which is not the same as the one that calibre uses for ePub2 ebooks. You would see something like:
Code:
<meta property="belongs-to-collection" id="id-6">The Hellspawn Brigade</meta>
<meta refines="#id-6" property="collection-type">series</meta>
<meta refines="#id-6" property="group-position">6</meta>
Code:
<meta name="calibre:series" content="The Hellspawn Brigade" />
<meta name="calibre:series_index" content="5.0" />
I suspect that MoonReader+ does not know what to do with that method of embedding series/series index.