So... not sure where to ask (or if I should open an official ticket), but here's the thing:
So I've decided to become Premium again. (I was last Premium back last October if I remember right) I go to select my plan, and instead of the $11.99 that should display for the amount I would be billedfor today, it shows $0.00. Continuing to the Tilia payment screen shows the same, $0.00 due. And this is for both monthly and yearly regular Premium plans.
If I go back and choose Premium Plus, everything correctly shows the amount I would be billed for today, for both monthly and yearly.
(And no, I most certainly am not going to try to take advantage of this glitch if that's what it is 🙃 )
I suppose this is support ticket territory after all, but I guess if anyone has any insight I'm all ears.