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Some of what you say makes no sense to me?

First, Smart Connect, and what is does when set. With it on, ALL SSID's share the SAME SSID and PASSWORD. You MANUALLY connect to that ONE SSID. The router 'supposedly' knows enough to put the proper devices on the proper band. Unfortunately, that is in theory. NG's implementation seems to be a 'load leveling' method. That means it will try to keep the same number of devices on each SSID. It is not a 'switching' implementation either, that it would dynamically load level. Once on a band, the device stays there.

In theory, all the devices the senses is faster will go to the 5Ghz band, and the slower to the 2.4Ghz band, until the number on the bands are uneven than it doesn't care, a device will be put on the band with the least devices.

That is what I discovered on the R8000 and so did SMALL BUILDERS NET that reviewed that router. More than likely all NG routers act like this.

Now you said Smart Connect was off.  With that being true, it can get 'dicey'.

You have to MANAULLY connect to an SSID the first time. If you had previously connected to both SSID's, that is where you can run into the devices being not on a band you want. Especially mobile devices vs. a desktop PC on a desk. The reason being the device usually will connect to the STRONGEST SSID signal. This means it might depend on where it was turned on and if it moved to the outer range of the SSID signals. The 2.4Ghz signal will generally travel farther than the 5Ghz. However, towards the outer limits of the SSID's, the 5Ghz will be stronger but not as far reaching as the 2.4Ghz SSID. Once on an SSID devices will usually stay there until it sees the signal to weak to use and will switch when it finds a stronger signal.

Basically Smart Connect will not solve this.

Best way to keep the devices on the specific bands you want. On each device you have determine which SSID you want to be on. Then on that device FORGET the other SSID. That will fix it, again, assuming the 5Ghz band is working properly. Note, house construction can change the strength of the signals, and the 2.4Ghz band is more likely to have interference weaking the signal as well.