It's possible to get the colony advancement 'Some Reservations' after creating only one park sign to make one room a reservation even though it specifies 4 reservations be created.
It seems to happen after building a park sign and then doing digging in the room to expand it. The room seems to flicker between 'nature preserve' and 'miscellaneous room' as the digs happen, which seems to count as more than one nature reserve being constructed.
Unfortunately, I deleted the screenshot but I did pull out a save file after it happened.
Build a park sign in a closed room (>32 tiles) with four plants so that it gets marked as a Nature Reserve Dig in room to expand Wait for Colony Advancement