I just found this thread because I moved my Slate AX from one location to another, when using it in repeater mode, and it stopped broadcasting 5GHz wifi. Yet the web UI claims 5GHz is turned on and active. So this is definitely still happening.

At both locations, the Slate AX was connecting to house wifi, and VPN was turned on.

At location 1, I think the Slate AX was connected to a 5GHz network. At location 2, it connects to a 2.4GHz network.

Firmware version: 4.1.0 release7

Edit: adding a screenshot from Luci: