As a data point, I'm seeing the exact same thing: I'm also trying to link some complimentary lounge passes for an upcoming JQ flight out of BNE in a few months time, and I notice the exact same issue:
View attachment 489245
It doesn't matter what date of travel you choose, it does not recognise/give you the choice, of choosing brisbane.
I assume its a school holiday/capacity thing, but it would be nice if it just said that, instead of making me think that I've spelt brisbane wrong, so I'll try again later in January to see if the restriction has been lifted.