I have also had this on all of the LE11 builds I have tried throughout December, prior to this I had no issues with the default renderer in LE11 (the DRM Prime renderer still does not render 544x576 broadcast video, ony audio)
I'm runing generic - x86 with an intel CPU and AMD rx550
I have noticed that changing resoluton sometimes helps, but the issue always comes back within several seconds: For me there are large flickering black triangles usually across the top of the display whenever the GUI or overlays like Player controls are rendering, these are more visible when pressing buttons on the remote.
The actual video streams for me seems to play with no issues (unless I use the playback cotrols) but I haven't tested that a lot, as it's my daily PVR I had to roll back to LE10 rather than test any further.