There's no sign up or registration for becoming an open tester. If you own Oxygen Not Included and want to join the Testing Branch, all you have to do is play the Open Testing build on Steam. How do I play the public testing build?
It may be a bug, but I think it's more likely that the pathing considers the airlock as being blocked by the water. I would try to build a small ladder at the top of your reservoir (to the left of your generators) reaching down into the pool, and it might make fix the dupe beha...