We’re in agreement; the variable here being whether a recording is going to take place on that channel or not.
Assuming there is no recording taking place and it’s just live TV - selecting the channel # vs the airing in the grid won’t produce different results.
That’s why the two most common reasons are noted above.
Not adding much value, but just adding I experienced this too last week on my 4 tuner Tablo and Roku 4.
I have almost the same setup (Roku Ultra and 4 channel TabloTV) you have and I have this same issue. I select a channel from the left hand side channel list in the guide (not the actual listed programs in the guide), and start watching this channel (because it is a favorite channel of mine). I am not recording any other shows during this time (usually during the day from about 8 am to 4 pm) but somewhere along about noonish most days, the tablotv just drops back to the guide (and I usually notice this because I am watching a particular program at the time). So I am interested in what the solution might be as well. Quite annoying.