Something is definitely going on there. Searching direct produces no results. The only way I have found this is as a married segment paired with a US-TYO flight, and even then it only shows up when using the award calendar search. For example, right now using the award calendar search I can find LAX-TSA on 3/6 via HND (LAX-HND, HND-TSA). Other partners show availability direct HND-TSA, no problem. Either Alaska has an IT bug (not unheard of), or either AS or JL is restricting availability through Alaska. I don't believe AS IT is that competent to restrict like that so my guess would be either an AS IT bug or JL restricting availability on that route only to Alaska. I'm leaning towards an AS IT bug. You should report the issue and see what comes of it.