Missing / wrong Results für seach string containing a German umlaut

[flyfoto]

If I seach for "büchelberg" (in double quotes for exact match) in https://fly-foto.de/luftbild/suche/
I get results matching the search exactly. See

However if I omit the double quotes the result including the search term is no longer returned but instead matches with 'b.'
see

If I search for "buechelberg" I get no results but if I search for buechelberg (without quotes) I get similar wrong results as above.
This does not happen if I search e.g. for "buchen" or buchen.
Here the exact match results are included in the result sets.

I suspect the Umlaut "ü" is causing this trouble?
However with the search term "Fürth" I dont get this unexcected result.