Dude Valguris don't post Beta 5 bugs here, Beta 5 isn't even out yet xD. We hash out prerelease issues on the chat, since that's the only place where people even know there
is prerelease testing. Putting it here can confuse everyone else.
DM does know about W, and the location--this is in the lore (DDarkray!), so technically it should be there, and I've also tested this before and see it as normal.
The question is when did you notice it no longer there anymore, and if there was something you did before that which may have affected that knowledge, but looks like all you really did was exit DM to the next level. But it was there when you looked while inside DM lair?
And in this same run, did it appear in the map
later, specifically after you exited the Proximity Caves and re-entered -5/Factory?
I would need a save from this run with a map like this to tell any more, like at least be able to look if the map is supposed to be known in memory and perhaps it was just a display error on your system? (unlikely, but just another route to explore)
To my knowledge you should 1) get that location, and 2) there should be no way to actually lose it there. I've tested it many times under various world configurations and possibilities.
The only way to lose map locations is via CR, and you of course hadn't done that yet
In any case, this is not a Beta 5-specific issue. Whatever happened is not new, just not something that anyone's encountered before...