I love the flavor that develops from lacto fermenting mushrooms. I'm a little bit suspicious of white button mushrooms (Agaricus species) since they contain agaritine. Agaritine is broken down by heat, making it safe. If you eat raw Agaricus, the agaritine is made into phenylhydrazines in the gut, which causes liver damage and can eventually make you anemic. Under no circumstances should you eat raw Agaricus.
So what about lacto-fermenting them? We know agaritine is heat sensitive. Is it also sensitive to microbial activity, so that it breaks down?
I ferment all kinds of mushrooms, cultivated and wild. Part of my job is to forage so I come across different kinds. Some develop very interesting flavors, others are just good and a few are terrible (Flammulina velutipes, for example). The most available mushroom we have is white button mushroom (Agaricus bisporus) and it gives a nice sour-salty, concentrated mushroom umami liquid.
All agaricus mushrooms, regardless of species, contain agaritine. This has been well-known in mushroom circles for a while and the official advice is to moderate your intake because it is potentially very problematic and doesn't totally break down when cooking, but raw is definitely there. https://en.wikipedia.org/wiki/Agaritine
Mycophagy is a pretty experimental endeavour still and there's lots we don't know. So the available knowledge can sometimes be scarce. Most I know I have directly from local mycologists and our fungi association that sometimes reports on the science. Fermenting mushrooms is even more obscure where I come from, so very few people have good knowledge on this.