What seems to happen is the toon that earned the final bit of rep that puts you over the 7500 required has the reward available, but any other toon the reward isn’t available. For example I was at 7460/7500 rep with council of dornogal, and did a WQ that rewarded 50 rep on a seldom played alt. I wanted to pick up the reward on a different toon, but for that toon it shows 10/7500 rep with council of dornogal, and no reward available.
When I saw this in the past I thought that the rewards were just disappearing, but it seems they are still available, you just have to search through every toon to find out which one has the reward.