Ever smell a mash that's been left overnight in a cooler? It usually has a bit of a lactic twang to it. I have done overnight mashes in my oven with the oven set at 170 degrees and that worked well.
i forgot to empty my mash tun one time for a few days (maybe 3 to 5). lol at that point it was significantly more than a twang. i think i posted here on how to get rid of the smell. it did work, vinegar, baking soda, PBW probably, i cant remember.
one data point indicating processes are for sure going on
I have done extended mashes (2-3+ hours) a couple times in my Anvil Foundry set at 150F or so, but never a full overnight, except when I once made a Berliner Weisse with some grain as the lacto agent (after regular mash, I brought up to near boil then pitched some grain and left it overnight until I reached my intended pH).
i actually really like 2- 4 hour mashes as it is a great way to really get high attenuation and i dont think it did anything to the flavour