I got a 4th gen tablo and on 2 of my 3 TVs (not smart TVs, use roku stick) the network channels come with picture only about a third of the screen and the rest is an empty green screen.
This is common with older Roku devices. How old are your Roku sticks? I have a similar issue with my oldest Roku Expresses. Usually, if I do a fresh re-boot of the Roku it will clear up the issue, at least for a little while. It may be time to update your Roku.
@KGBnut wrote about this last year in this thread/post. I think it boils down to updating the Roku firmware or getting new device(s).
Picture size un screen - Support & Troubleshooting - TabloTV Community
It’s a known limitation of low-end Roku models. We have the details here: Video Does Not Fill Screen, Shows Green Border on Roku – Tablo
With all the money
you save with OTA and Tablo TV, get a higher end model like the Roku Ultra. I’m glad I invested in the latest model. Made a heck of a difference.