This sounds like some slight missunderstanding about how Random Maps understands "layouts" and "exits".
Layouts manage how all the maps are connected to each other, see: https://aerosys.blog/room-layouts It does not affect how the small "rooms" on one generated map look like. Instead, 5x5 means that Random Maps will pre-generate 25 maps in total that are connected in a 5x5 grid. Meaning, that there ARE horizontal exits in each map.
To only enable vertical exits, the simplest way is to use a "tower" room layout.