If I execute a WFS GetFeature request on a QGIS Server, specifying outputformat=application/json and srsname=EPSG:32633, I obtain result in EPSG:4326; if I omit outputformat to obtain result in GML format, the coordinate are returned in requested CRS. I have see that on WMS GetFeatureInfo request there are similar issue (https://github.com/qgis/QGIS/issues/32326) that are marked as resolved to return coordinate in CRS requested from client.
I'm asking data in JSON format, not in GeoJSON (that has application/geo+json as format and which requires that coordinates to be geographical).