When attempting to use spread players for a PVP map, I noticed that is spreading to y 257 and not on top of a block. The command always spawns the players at y 257 and never directly over a block, always over the void (The world is an air world). The commands are nearly the same for every course, but one is:

/spreadplayers 1534 9 5 50 false @a[score_course=7,score_course_min=7]