I am having the exact same problem with my Roku Premiere+ 4630X with version 8.0.1, build 4090-29. I am also on Tablo version 2.2.18 on a 2-tuner Tablo.
I do NOT have this issue on my Roku TV 7201x it is on Software 8.0.0 build 4183.38. I have tested this with both devices on the same program at approximately the same time.
I don't know when this started, but I did notice it right after I started the Tablo app, and it had to reconnect with the Tablo. So, I am assuming that was a tablo firmware update that triggered the reconnect???
Just for clarification, this only happens when trying to rewind a Live TV recording (i.e., watching a show on live tv, while it is being recorded, or just the Live TV buffer). 'Play' or 'OK' has the same effect. If I select the show from the recordings menu and watch it, I can rewind, fffwd, etc., and pushing either Play or OK will play from the point selected.