Some further reading and experimenting since posting. My Windows laptop "sticks" with 2.4GHz, unless I manually change the laptop to use 5.2GHz with the Intel tools. My wife's Linux laptop automatically went to 5.2GHz.
I have a couple of devices that are 2.4-only, so I actually don't want to turn off or down the 2.4GHz.
At least I know I'm not going crazy.