Dryland terrain can't be painted down touching desert terrain, in any way. What you end up having to do, if I remember right, is blank out the entire map with water, cliffs, or chasms, and then replace those with the drylands tiles.
Edit: It seems that this only works using chasm tiles, at least with the version of it that I have, which is likely 2.3 or 2.4 only. Desert and dryland tiles can be connected via a bridge crossing a chasm tile. The chasm itself could then be covered with water placeables, cliff placeables, etc..., to hide the seam if desired.
The other alternative is outright tile hacking it to place desert and dryland tiles together, though this can be extremely tedious given that the map will be very hard to manually adjust afterwards.