I have the same issue.
I failed a 4/5 Boar hunt, but immediately got a prompt that a Legendary White Wolf was sighted in the same County and would be available for 3 years.
The clock ticked two years to the point where I could stage another hunt, and I see the same thing as you - White Wolf modifier is listed in the County as available, but in the Hunt activity menu, the Legendary option is grayed out and unavailable.
Unlike a usual animal sighting, it also doesn’t show up in the “Current Situation” menu - the only record of it is as a modifier to the County itself.
Devs, if you need another save to compare this one to, let me know and I can upload it.