Eliette's journals explain her descent into madness. She fell in love with Lolth, then felt Lolth had sent her an arachnomancer who taught her the transformation magic. So she returned to Moonhaven after being gone for a long time. She only escaped the Drow after some fool created chaos there. So MANY years went by.
All the writings imply, Moonhaven was laid to waste in 1371, 121 years before BG3 began. Yeah, the poster still being legible is odd, but when you put everything together it seems the entire Druid Grove murals, Moonhaven events, etc. all coincide with one another to form one cohesive tale.
Besides, I highly doubt anyone would have been living there with Eliette prowling around. The apothecary and blacksmith shops have also not been used since 1371 either. So...
