I had all 4 5009s running off a Cisco 3850 using PoE. I bolted everything together with the rack kit (shared ground). Powering one works, two units mostly works, connect the 3rd and the PoE to all of them shuts off. I’m guessing this is a PoE detection circuit the shared ground/chassis connections is screwing up?
So, you can power the 4 RB5009 from that power source when they are separated but then, if you make them sharing the ground (by screwing them together to the rack) the PoE PSE powers off?
Since the issue happens with just three of them, does it happen with any three of them?
I.e. could it be a defective unit or it happens anyway no matter which three units are involved?
I wonder if it can be an issue with resistance detection on the ethernet cables, but having a common ground (but separated + ) shouldn’t cause any issue at least in theory, it is not like you are putting resistors in parallel, they are still “open ended” on the + side.
It is more likely that there is one (defective) unit that leaks voltage towards ground and alters the reading/check, but still in theory only one or maybe two units would be affected, not all of them.
While you are doing these tests the rack kit mount is or it is not connected to the actual rack (and thus to a “real” ground)?
Could it be a remake 
, some 40-50 years later of the classic magic/more magic switch?
http://catb.org/jargon/html/magic-story.html