Spatial filtering help needed

[PB-GNM]

Hi,
I have setup a localhost server with Pygeoapi (0.21.0) with a geojson file as source.

The API works fine, but it does not seem to react on any of my spatial filters according to Part 3 of the filtering standard.
In all the cases, it returns all the records (567), instead of the 2 or 3 that I expected.

http://localhost:5000/collections/Aandachtsgebied_EV/items?filter-crs=http://www.opengis.net/def/crs/EPSG/0/28992&filter=INTERSECTS(foo_geom,%20LINESTRING(138000%20523000,%20138000%20525000))
http://localhost:5000/collections/Aandachtsgebied_EV/items?filter-crs=http://www.opengis.net/def/crs/EPSG/0/28992&filter=CROSSES(foo_geom,%20LINESTRING(138000%20523000,%20138000%20525000))
http://localhost:5000/collections/Aandachtsgebied_EV/items?filter-crs=http://www.opengis.net/def/crs/EPSG/0/28992&filter=DWITHIN(geometry,POINT(138000%20524000),1000,meters)

I have also tried it with WGS-84 coordinates, without filter crs, but that does not make a difference.
Does anyone have suggestions what could be wrong?
Are there any preconditions, I should take care off?

Thank you for looking into it,
Pieter