My memory of the FN-49 is a little hazy, but doesn't the bottom curve of the bolt carrier & the tunnel for the firing pin in the bolt carrier prevent a following hammer from contacting the firing pin till it is 99% in battery?
In the FAL, which is a development from the 49, the hammer may follow through, but can't contact the firing pin rear till the bolt has dropped into the locking recess & the carrier moved forward exposing the firing pin.
If I had to guess I'd say high, or defective primer, or jammed pin in the forward position.
Allan Quatermain: “Automatic rifles. Who in God's name has automatic rifles”?
Elderly Hunter: “That's dashed unsporting. Probably Belgium.”