I’ll put info in spoilers so dont read if you dont want your “immersion” ruined.
The brown/grey F is indeed a Demon Spider Queen lair, and it’s specifically tied to Magiclism, also being a very “popular” location - which makes me really surprised that no one who commented here knew about it.
And no, the [F]orest being a different colour isn’t a glitch/oversight/bug: it was made to be clearly distinct but not too obvious to an obviously easily distracted player - the same way there are specific map tiles that represent two completely different things, but have the same name and colour, except for a Capitalized letter on their name (not overmap tile. Actual name.) Imagine if there was “house” and “House”, for example.
They are indeed somewhat nasty creatures - the spiderlings are just a nuisance, as they dodge a lot, but the Demons are much scarier because of their health and their (still OP imo) magic abilities and defences. And if you really think they are the worst, then do yourself a favour and don’t try to seek out the Spider Queen. You won’t like the end result.
As for the suggestions left above, only one advice really works against them - the obviously realistic one - whereas the rest are just a waste of time and assured death: they can range attack you, the spiderlings will foil your trapping plans very quickly too.