Last night I had 3 shows recording at the same time.
Modern Family on ABC
X-Files on Fox
Riverdale on CW
while they were recording, my TV displayed a tuner has been re-assigned.
I flipped to live TV and scrolling up and down. I see Brit Floyd on PBS. I select it and I get a message that there were no tuners available. I could select one of the currently recording programs or previously recorded program.
I have a 4 tuner Tablo, so I should have been able to use the 4th to watch a live tv show. Why not?
I checked the live tv guide, and only the recording stations were blinking red, indicating the tuners were in use. No one else has remote access to my tablo. Is there a way to see how many tuners might be in use and from what device it is being used from? Future feature?
https://www.tablotv.com/blog/tablo-tuner-math/
However, there’s more complete info “somewhere” that includes how an extra tuner is used on the highest quality setting in certain cases.
Ugh, my explanation was only partially correct, so I removed it.
The Tablo Support article in the comment below accurately describes what’s happening.
I verified it on my Tablo Quad.
That’s on the KB:
An updated ‘tuner math’ article is on the to-do list that includes info on corner cases like this that have popped up since the original blog was written many moons ago, in a land far-far away. 
Also note that a 2-tuner Tablo can record two 720p shows at the highest settings. These restrictions only affect the 4-tuner Tablo.
Majing thumbnails might enter into it too, unless thimbnail geneeation gets suspended when tuner is needed