A pot roast, to me, is using a casserole dish or roasting pan, adding carrots, onions and potatoes, and baking/roasting at about 350. Mushrooms are like little sponges and will absorb the flavors of other ingredients & become limp. Here is a general description of same.
https://en.wikipedia.org/wiki/Braising
If you want to add mushrooms in a pot roast dish, you might consider adding them at the end of whatever process you are using.
Your call.