Just had this happen again. next time there will be screenshots!
I was watching a show and noticed no close captioning. I thought it strange, because the show should have CC.
Quitting out and restarting the show restored closed captioning.
If you are using Roku, they have been "improving" the CC engine over the last few firmware updates... I reported some issues to them and saw them improved in future builds just to see them go away again.
The most recent build I am running hasn't lost Tablo CC in the last 48 hours. Fingers crossed they got it fixed.