Nice work. Devices are supposed to connect to the 5 GHz signal when they are close to an AirPort and then switch over to 2.4 GHz when they move a room or two away from the AirPort.
Even if they connect at 2.4 GHz, they still should be connecting at up to 130 Mbps speeds when they close to the AirPort, so I have no idea why you were having issues with only one AirPort at a given location when things worked fine at other locations. I suppose it is possible that there was some sort of weird interference issue with 2.4 GHz at one AirPort location, but this would be rare.
That's the way things work here, but no two networks are exactly alike.
Personally, I don't like having to manually tell devices whether to connect to the 5 GHz named network or the 2.4 GHz named network, but some users don't seem to mind doing this. If you are OK with this, then it sounds like you have things solved until the next mystery appears.