Hello

English is not my native tongue either...
Yes you are right when metada field is located after the series field , Pocketbook loose the information.
I opened another bug on this but Kovid told us that it was too complex to change all the metadata fields and advised to go back to Pocketbook.

We (from a French forum) picked up  the name of the marketing guy in France ... as the support in France tends to answer all complex requests by "reinitialise"  and sent him an email  in September. We were contacted  back to provide samples. No other news so far.
I would advise you to send similar requests to German support... and the more the better.
German market for pocketbook is much bigger than the one in France and Pocketbook might put more stress on the request if it realises that it is important for a lot of people.

Here is the email we sent (more or less)

Dear Sir,

I had the pleasure to meet you 18 month ago at the "Salon du Livre" in Paris and we had a really nice conversation about the Pocketbook.

As I am part of a forum group which gather thousands of readers (and pocketbook lovers!) you gave me your business card and asked me to send you any feedback our members could have about their use of the Pocketbook or any bug they could face.

This is why I would like to report you some bugs that we find really inconvenient.

Most of us have chosen the pocketbook not only because it is an excellent e-reader but also because it can be an excellent e-library for large amount of books.

Unfortunately, we detected some problems with Pocketbook software in the way it collects metadata in epub files for indexing the e-library.

The software seems to be unable to pick some data in the opf part of the epub file, if the fields are not in a specific order.

To get a little bit more technical, the series information is lost if the meta element with name="calibre:series_index" is located before meta element with name="calibre:series".
The software also seems to ignore meta elements with name "calibre:series" and "calibre:series_index" if "content" attribute is located before "name" attribute (e.g. <meta name="calibre:series" content="test"/> is ok while <meta content="test" name="calibre:series"/> is ignored.
Also, "dublin core" (dc) elements (such as "dc:title", "dc:subject", etc.) located after meta elements (such as the series ones for instance) are simply ignored.
To sum up, analysis for ebook metadata retrieval is unnecessarily strict with opf xml structure.

This behavior is the same from PocketBook Lux 1 software FW 4.4 to Pocketbook Lux 3 FW5.9

We tried to get around through calibre but received a "wont fix" from Goyal for technical reasons. He advises us to go rather through a Pocketbook firmware update.

The truth is that the small free add-on application named "epubinfo" is always collecting perfectly all data whatever the opf metadata elements order is. And it might be also a good idea to integrate it into future release as the summary of the book it gives (DC description field) is useful for all of us.

We noticed that since the lastest firmware releases, Pocketbook is able to handle more than one genre per book (which makes it unique among all other epub readers!!). It 's all the more a pity that as the library improves, the extraction of the data to build it is still buggy.

The library management is something that makes Pocketbook stand out from a Kobo or Kindle, and a selling point for some of us.

Thanks in advance
Best regards