Mushrooms, beef, long cooking and bitterness
Ooooh!
I would not recommend using cast iron for slow cooking or long cooking of any kind. Especially if any sort of acidity is introduced. That bitter taste? That straight up might be the taste of the pan corroding. Cast iron is great for fast frying meats, sauteing vegetables, but it is not recommended that you make anything that has much liquid, and definitely avoid acidic foods. This is why it is always important to wash and dry your cast iron as soon as you are finished cooking in it - to avoid rusting and peeling.
Additionally.... I would try sauteeing your fresh mushrooms first in a bit of butter, over high heat. you want to sear them. Then add them to your soup :) the full umami (salty) flavor should come out nicely!