Wait, all this time you were healing the Merc via the menu?? On PS5 it's hold L2 and press whatever directional button with potion you want to give, absolutely essential for Merc survival, I can see a lot of people having their Mercs die quick if they aren't aware of the shortcut, good you posted for people who might not know.If you're on singleplayer (Nightmare or Hell) and want to refresh the map for whatever reason, you can do so by going back to Normal difficulty then Nightmare or Hell again. I just went back to Normal to get some chipped gems and all of the areas were refreshed. Same for when I went back to Nightmare after.
Although it's right on the help menu, using shift + 1-4 uses up the potion on your merc automatically. Instead of having to drag and drop it over their menu on the top left corner.
And yea, you can reset the maps by doing that. Biggest utility is resetting the *enemies* though. Since both the map layout and enemies are randomized it resets both, if certain enemy types are giving you trouble try a map reset and they might not even appear in the reset map. It was weird when I was playing through Nightmare because there were so many situations where I said "wait, I've never seen *this* enemy type before!" on my Normal playthrough, that's the reason, was pleasantly surprised by all the new stuff lol.