I checked all modes, and pretty sure this is limited to the Tablo Channel on Plex. My wife likes to watch The Today Show, starting about 8:30 in the morning. The show is listed in the Recordings section, but it is really the same as Live TV: it does not play from the beginning, instead from the instant time of the live broadcast, which starts at 7:00am. It cannot be forced back to the beginning. Checking live with another unit, the “recording” is indeed the live feed. This is the same for Plex / Tablo on a (very old; Rev 0) iPad, on my Android (Lollipop) cell phone, and on the Plex / Tablo channel through an old Roku N1011. But the Tablo native app and web app are both fine, so it is strictly a Plex / Tablo Channel issue.
Now it is more odd. I thought that Plex gets a command to start and then just ships the video open loop. The above problem (can’t watch an in-progress recording from the beginning) is manifest on an old iPad with Plex, playing the Tablo Channel under Plex. But on a TV connected to an old Roku N-1101 with the Plex Channel, and the Tablo Channel under that, there is no problem. But why will Plex start at the beginning on one and not the other? Same Tablo Channel under both.
Plex Tablo channel is a 3rd party app so you will not get an official response.
But that is odd of the differing starting positions.