There isn't anything in Lightroom to configure the keyboard. Since there will be no more updates for Lightroom 6, it seems to me that you'll have to assume it's a problem with the keyboard or with your operating system. I don't know of anything you can do in Lightroom that will change the behavior.
Is there a different character printed on the L & R arrow keys along with the arrows. If there is that is tell you it has a Dual Function and to activate the Dual function for one of those you have to also hold down the Fn key.
This is a Setting in the OS or in some keyboard software to change the function from secondary to primary or the other way around.
There is only one symbol (arrow) printed on those keys. All the arrows worked in LR5 so it must be some idiosyncrasy with LR6. I had to upgrade my OS in order to install LR 6 so maybe it's a problem with the LR6/Sierra combination.