Then it's the same app/channel, exactly what I said. I already mentioned that Roku has the ability to change aspects of their UI on different devices, and that's what you're seeing. I have no explanation why a video plays on one Roku device but not another. But it's not that they have two different versions of the app, because they don't.
Remember one thing about a Roku TV compared to a player. The TV OS has always supported MPEG-2, since that's the North American broadcast standard with ATSC 1.0. Roku didn't add MPEG-2 support to their players until the patents expired on MPEG-2, so they didn't have to pay the license for it. Even with the "same" OS versions, TVs still have a slightly different OS than the players. So the issue might be with the OS, not RMP.
As I mentioned before, I don't have any version of the Express. I do have two Roku TVs, three Ultra players, a Premiere (3920), an Express 4K+ and Stick 4K+. Every one of these will play MPEG-2 videos without a problem. My Premiere is the closest hardware to your Express, and I just tested it yesterday to make sure it still worked.