Coincidentally I was planning future travel today for December and January which included January BKK-HKT and HKT-BKK. I didn't notice any problems with flights not being available. But I did notice that things have drastically changed since the last time I booked this route. There's now basically only one fare available for economy and one for business class. And then for flights involving Thai Smile, there's one additional cheaper economy fare. That's it. There used to be a whole slew of different fares with various conditions in economy and two selections in business class. Not just BKK-HKT, but for the routes I was looking at it seems to be an across the board change for all domestic flights.

I imagine they'll still have some specials from time-to-time, but at least for now you're very limited in the fares you can select from, at least via TG's website. Not just for next January, but for all flights on all dates.

Edit: I took a closer look, and even though they list the fares in just a single column, they're different booking classes. This is weird. For example, on a particular date, one fare was 3015THB and booking class WOWTH05. Another was for 3705THB and HOWTH05. Perhaps they're only showing the lowest fares in each column and incorrectly labeling it as all one type of fare when it's not, and not allowing the customer to select a particular fare for a particular flight. Not sure if this was what's intended or if it's some major bug with a software update. I guess for me actually though, being I'm cheap and want the cheapest fares usually, it's no problem for me personally. For others though, who want a more flexible booking class, it's going to be a huge problem for booking on their website.