This is happening to me as well on all 3 of my Express 4K units. They are only a couple of months old so not related to software updates, unless they updated during the set-up process. I use Roku to access the Spectrum app and it happens on all channels. If I switch to another channel, the problem clears and I can go back to the channel I was watching. Does not seem related to WiFi strength as one of the units sits right next to my router.
I did not read every reply here as there are 17 pages. But it appears that the problem is with the Express 4K. I need to go back and see if the standard Express units have exhibited the problem. If not, then these are going back and I'll get the Express units or maybe consider a different brand entirely.
I also found it very disappointing that for customer with 3 brand new units, there is no agent support. It would appear that Roku is not interested in customer service. Another reason to change brands.