highlighting section nearly empty

[solr]

Hi all,

I'm running Solr 9.1.1 on Windows 10 Pro.

Search is fine, but the highlighting section at the end of the results 
does only show the documents UID.

I search for 'penke' and the string 'penke' should be marked for 
highlighting.

Any idea on this?

Best,
Walter Claassen

Query URL: 
http://localhost:8983/solr/core_qst_2023/select?q=penke&hl=true&hl.fl=*&wt=json

Result Array (converted from the original JSON and reduced to the 
minimum):

[responseHeader] => Array
    (
    [status] => 0
    [QTime] => 11
    [params] => Array
        (
        [q] => penke
        [hl] => true
        [hl.fl] => *
        [wt] => json
        )
    )

[response] => Array
    (
    [numFound] => 10
    [start] => 0
    [numFoundExact] => 1
    [docs] => Array
        (
        [0] => Array
           (
           [UID] => dgiz05-1993-02-nfd-0058
           [PARENT] => dgiz05-1993-02-nfd
           [CCODE] => dgiz05
           [SCODE] => Array
                (
                [0] => dgi
                [1] => iww
                [2] => qst
                )
           [DCODE] => 1993-02-NFD
           [DPATH] => collections/dgiz05/1/1993-02-NFD.pdf
           ...
           [PTEXT] => Programm der DGD-ONLINE-Tagung
                            ...
                            15.00 Organisation, Datenbankangebot und 
Kostenverrechnung der Informationsvermittlung an der TU Berlin; K. Penke 
(TU Berlin)
                            ...
           )

...

[highlighting] => Array
     (
     [dgiz05-1993-02-nfd-0058] => Array
         (
         )