My model is 3941X Roku Express 4K+
SOFTWARE OS VERSION 11.5.0 BUILD 4235-CR
The problem occurs with Xfinity Stream version 6.9 build 0 and maybe others.
The video will freeze after 5 seconds or so, but audio continues, but ocasionally video will work fine, if it makes it past 5 seconds or so. Power has been from USB on TV and been fine all along. I tried using an AC adapter with 2 amps USB capability plugged into a wall outlet and that fixed my problem.
Something in your recent upgrade requires more power than the TV can deliver.
Two TVs work fine with 1K Roku sticks powered off the USB on the TV, but both exhibit problems with the Express 4K+ when powered off the USB on the TV. Both work fine when powered off the the 2 amp USB wall adapter.
Hopefully, you find this helpful.