Sorry that you got no reply in more than a year, but here’s an attempt.
From reading the recipe I think there may be something wrong with it. Evidently the preparation lacked body.
Moisture is usually not an issue with mushrooms unless you soak them in water. The way with cèpes and shiitake (oyster mushrooms rarely need cleaning) is using a damp paintbrush and/or damp tissue paper. But if cèpes were picked in rainy weather they can contain plenty of water. However I doubt that’s the real problem there, I’d suspect the forcemeat (pork + breadcrumbs) wasn’t firm enough. Normally it shouldn’t need one extra egg.
Would it be possible to have a picture of the recipe in French?
