I've had a new ISP since last August (Fiber Through the Air Technology, FTTA). This aspect-ratio-flipping didn't start for me until the last month or so (so, early 2021). My speed tests have shown no network difference between late 2020 and now, early 2021.
It happens to me on all channels, not just On Demand. But it only happens when viewing via Dish Anywhere. (In other words, only inside a Web browser, not when I watch the TV directly.)
I use DISHAny from 2 Windows notebook computers on my home network, and this flipping happens on both of them, around the same times. It doesn't seem related to buffering, because it happens throughout the program, not just at the start. It happens with both live viewing and DVR playback. It's very annoying.