Sorting with Series # not working correctly

[rift-henna]

Hello.
I have a weird issue with a new series of books I want to cleanup and order properly in calibre.

I have added the books of the series and drag&drop them together into one specific series. So all of the books now appear within this specific series.
Ordering however needs some work. Here I am encountering issues.

I have corrected the sorting # of 4 of the books. Strange thing now is, that no matter what I do, the book with sort # 5 always will appear first prior to the books 1, 2 and 3.
I have checked with both latin and roman numbers setting and w/o setting to consider numbers for sorting - athough this should not affect sort number field anyways?
It does not even matter which number I give this book # 5. If it has # 235 it also sticks to the first position although most of the other books have lower numbers.
I tried removing and readding the book. I have checked the series and numbering for any trailing empty spaces or so.
Nothing works. This specific book always sticks at the first position.

Any ideas would be highly appreciated.

Thx
Rob

[rift-henna]

Ok!
I think I fould something just to react on my own question!

When I name the series "Test", the book appears in the correct position. But when I name the series "The Test", (so with an article) this seems to not work properly.

Obviously it does not matter whether it is English or German. As soon as the article is added, sorting is weird.
But it seems that I have different behaviour here with articles within series names. Some of the series with articles are added within the first letter of the article, other series names are added to the first letter of the noun after the article. So a series with name "The Last Man on Earth" should appear at the letter "L". But this is not the case with every series.

Why is this handled differently? Is this something I have to manage my own or can I cleanup the series somehow to handle articles properly?

[rift-henna]

Ok.
Think I found the solution.
I need tho assign proper language to the books. So considering (ignoring) articles of series names seems to work, when I set the books language to German or English.