Last atmo suit dock in line in rocket not usable if Clearance: Vacancy Only

You need to configure the dock to "leave empty" instead of "deliver suit" for the dock to be usable.

Oof, yeah, I see that's the case for those second two pictures, my mistake. I'll try again to do it with three docks. 

But look at the first pic, in that one you can see that there's no status icon indicating that dock 2 is unconfigured, I don't remember configuring it when I added the third one, and it's holding a suit now.  I'm still reasonably sure there's something going on here. 

I'll experiment further...

Welp, I can't make it happen now. That's weird. This has been plaguing me for a while. Could it be as simple as a bug in the status indicator, I wonder.

I guess I'll just have to be double sure that the dock is configured.  Something to add to my pre-flight checklist.

I've run into issues with docks inside the rocket as well and I'm fairly confident it's not due to leave empty / deliver suit. It seemed to be related to dupes idling on top of the checkpoint which apparently prevents other dupes from entering and using the docks. I've been able to 'unblock' the checkpoint by moving the idle dupe away manually.