My apologies for the delay, it looks like you might have already identified the issue, but just to clarify and answer, since I originated this post.
Line shows up usually immediately when the content starts.
I can't identify any specific content such as 4K or HDR, just happens on all streaming regardless of version. I did have Game Mode enabled, but not sure if the issue was present only when Game Mode was on.
I haven't been as active on this thread because, in all honesty, the issue seems to have disappeared for a good while; I haven't really seen the issue arise on streaming apps in roughly a month, and I use them on average 3-5 times a week, for about an hour or two at a time.
That said, I am wary that the issue could always come back, so I'm glad that, with assistance from Danny, Roku seems to have identified the issue. I similarly got a response from the Roku Support Team on my ticket via email, indicating separately that their engineers have identified the issue, so that seems to be confirmation on the ticket side that they've found the issue (finally), and is probably related to all of our input on this thread.
Danny, please let us know when we can expect an update to have this issue resolved, and what update that will be. Thanks!