Bob did you figure this out?
I have what I think is an identical problem in my 2013 GD Tourer, the centre hi stop led works, but two tail lights arent.
Checked globes and they are fine, but not getting brake light power at the tail light connectors. Tail and reverse lights work fine.
Googling the problem brings up a few threads which indicate its a somewhat common problem, but not many solutions.
One mentioned Hyundai did an ECU reset, whatever that means (or if its just what they told the customer they did to fix it).
Currently have the battery disconnected, but not real hopeful as I read others have tried this without success.
OBD2 scanner didn't detect any issues.
Would be handy to know the cause and fix, if it occurs again.
I did recently install a towbar, but everything was working fine after that.
I'm wondering if connecting a trailer on the weekend has caused it.