The database behind the service might have to update the extents of the featureclass.When you add a big boundingbox to the query it works:

https://carto.sigena.fr/cgi-bin/mapservwfs?&service=wfs&version=2.0.0&request=getfeature&typenames=n_enp_apb_s_r75&BBOX=-9589968,-3586633,10633187,16635814&resulttype=hits

numberMatched="78"

If you add this boundingbox in FME it returns 78 features as well:

ReadWfs2018.fmw

Found in QGIS when the checkbox "Only request features overlapping the view extent" was marked and zooming out I got more results.