I tried two long-cooking beef dishes starring mushrooms, an ossobuco of veal and beef breast. The directions went by basically cooking the ossobuco (after cutting the junctions) or the beef breast cubes in vegetable stock, not submerged, for some hours until tender.
Then I was supposed to add mushrooms and cook for another while. Champignon in the case of ossobuco and girolle in the case of the beef breast stew.
They did hold their shape after at least an hour more, but the resulting dish was in both cases bitter. How can this be related to the mushrooms?
I can't identify other possible causes for this. Both dishes included also a splash of wine, but in both cases, it had not turned into vinegar. The stew did contain a bit of rosemary but dry (and the ossobuco was way bitter anyway).
The cooking vessel was a steel pan for the ossobuco and a cast iron pot for the stew.