Here was the information I received from Tablo Support on 11/15/17:
"Since we started investigating this issue, we believe we’ve managed to find the root cause of the issue.
The two channels you’re having issues with (21.1 and 13.1) are the only channels broadcasted at 720p in your area.
Because these channels are broadcasted in 720, and you have the 10Mbps recording quality enabled, the Tablo is transcoding the video for these channels to 60FPS. This appears to be triggering a player failure in the Roku/Insignia branded televisions exhibiting the issue.
Unfortunately, this lands as a hardware of firmware failure on the television’s end, and we can’t pinpoint exactly where it’s failing. We’ve passed along this bug report to our contacts at Roku.
In terms of how to avoid the issue in the future, we recommend changing your Tablo’s recording quality to 8Mbps to avoid triggering the issue on the Roku TV."
So, the current status is that Tablo simply doesn’t work on some Roku TV hardware. Question for Tablo: Would it be possible to allow recording quality to be set at the highest level but have the Tablo cap the recording quality at the detected broadcast resolution? Or, to set different resolutions on different channels. Either of these options are better than accepting a lower resolution on ALL channels, or living with playback failures on some TVs.
Harry