I have a Tablo with 2 channels (Dual Lite), Version 2.2.24. I never had this problem before so I just want to check if you guys also experience same thing recently. I only use 1 Roku to watch Tablo at the moment and the TV shows that one channel is being recorded. But when I try to watch a Live TV, I receive an error: No Tuner available. I have not tried to restart the device yet, maybe I need to do it. But it seems strange. Normally, it should happen when 2 channels are being recorded.
Are you 100% sure 2 shows weren’t being recorded?
Could someone have been watching your Tablo remotely? Tablo Connect uses a tuner when the Remote Streaming Quality on said device is set to anything lower than Full Quality.
Correction… this only seems to apply to the OG 4-tuner model… that I have. 
The Tablo will use 2 tuners (actually, it just disables 1 of them) to record 1 channel under the following conditions:
Recording Quality set to HD 1080 - 10 Mbps, 720@60fps.
Channel being recorded is broadcasted at 720p.
Extra tuner is available.
The 2nd tuner on a Tablo DUAL Lite should still be available even at the 720p 60 fps recording quality.
“ And a caveat… For the 4-Tuner Tablo ONLY when using the highest recording quality (HD 1080, 720 @ 60fps) Tablo’s fourth tuner will be temporarily disabled if any 720p stations are currently being watched or recorded at 60 fps. This is due to the high processing requirements of this setting.”
Yes, 110% sure. I am not new to the Tablo so I know how it works and what’s I am talking about. It works after I unplug the power cable and plug it back in.
Why do you want to use 2 tuners to record 1 channel? Anyway, if there are two recording bars in Live TV guide, then you don’t have any available tuner. My case is different, only 1 recording bar (meaning one tuner is recording the show), then I should be able to watch any one channel I want. It does not work that way, that’s why I am reporting.
Anyway, it came back to normal when I unplugged the power cord and plugged it back in (power reset).