I above a Roku Express 2017, I have it plugged into a Samsung HT-F5500 which is for my surround sound.
The problem I am having is that after a few minutes I get a snowy screen which alternates between grey and green, the snow is made up of multicoloured dots all over the screen, the sound continues to work and if I switch source and back again, the display comes back.
I tried finding a manual for the Samsung HT-F5500 without luck. It sounds like there's some issue with your HDMI in on the Samsung, but without the manual I can't see if there's any setting that might be available. The fact it works correctly plugged directly into the TV indicates the Roku isn't the problem. Any chance you can get your hands on a different AVR to try and see if it works?
Dan Roku Stick (3600), Ultra (4640), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Nvidia Shield, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.