I am not sure, but try removing the "use default prefix/suffix" because, in my game, the "." is not occurring.
I tinkered with this when trying to make my title screen. Couldn't figure it out either.
The "." is hardcoded. If you want to remove it you must import the function ShowRoomDescription into your game (choose "Filter" in the left lower corner of Quest and activate "Show library elements". Then search this function and press the copy button.
Then change to code view and search this line
desc = AddDescriptionLine (desc, youarein + " " + GetDisplayName(game.pov.parent) + "." )
Remove the
+ "."