This problem usually happens on old MTA maps that have been converted to the current format.
Some converts just skip the 'rotation' property of spawnpoints that had the rotation of 0...
IMO the best way to fix this i to find the line in race_server.lua
vehicle = createVehicle(spawnpoint.vehicle, x, y, z, 0, 0, spawnpoint.rotation, plate:sub(1, 8))
and add this just before that line:
if not spawnpoint.rotation then
spawnpoint.rotation = 0
end