This is my setup for going into space and the only reason why i have docks on 2 sides is symmetry. Both checkpoints are set to "vacancy only" and the whole system works just fine, no dupe drops its suit on ground (unless worn out)
And here's my set up for going into space on another asteroid (oily one) and it is basically the same thing as before except now instead of suits we have masks. Just like before both checkpoints are set to "vacancy only", yet my dupes constantly try to go through checkpoint without any docks available and drop the mask.
Build the same setup as i have and let dupes in and out for sometime. If it doesn't work for starting asteroid, try different one (maybe it has something to do with asteroid)