By the time mushrooms enter the conversation in a beef bourguignon recipe, the heavy lifting of layering the dish's flavors is mostly done. The beef has browned and braised, the veg has caramelized and come together, the wine has reduced, and the meat's collagen has melted into the sauce. It's a long, gentle cook, in which mushrooms are lost if they're added too early.
Smearing a tomato-pasted finger across the page of the cookbook, trying to follow the instructions as the hot oil shimmers in the pan, ready for the onion you don't have brunoised. Quick! Turn off the heat, grab a (hopefully) sharp knife, (carefully) rush through the prep, turn the burner's flame back on, and hope nothing scorched or became soggy in the interim.