There seems to be an issue with expected language fall back behaviour relating to the correct language response per locale specific request.

typically a request for content should be returned in the correct language_locale, with the appropriate fall-back mechanism as follows if requested locale not found:

Request --> de_lu ... try to return this language_locale, if not found fall back
de ... try to return this language, if not found fall back
en ( default language ) ... requested lang_locale not found so returning default

currently this is demonstrating incorrect behaviour as follows:

Request --> de_lu ... try to return this language_locale, if not found fall back
de ... FAILS TO CHECK!
en ( default language ) ... just returns default without checking