[Klecks]

Hm, just updated my pocketbook reader to actual firmware and did some tests.
Calibre is still shifting around the metadata, but the non-dc metadata (series-info) are always under dc-metadata. So it shoudn't be a problem.
I deliberately shifted the series_index line before the series name and after transfer to the reader the book missed the series-index. (as expected)
Then I twisted the series-metadata (content="" first, followed by name="") and that book showed no series-metadata at all (on the reader).
So the pocketbook-firmware really doesn't like twisted series metadata.
Problem: Sigil does this twisting (content="" first, followed by name="") regularly - every time you open a book with sigil, the metadata are twisted.
And Calibre doesn't care. If you later load the book (with twisted series-info) on the reader via calibre, the metadata stay twisted. (the polish book function also doesn't correct twisted data).
Solution for that problem is: if you touched a book with sigil, you should use calibre and the modify epub-plugin and delete non-dc:metadata. After that you can use polish book for re-adding the metadata.

Klecks.