I no longer subscribe to Netflix, but it was my understanding that the refresh rate switch was disabled with the 9.0 release, because of all the complaints about the way Netflix constantly switched their refresh rate between the preview videos and the actual content. But since you appear to be in the UK, perhaps it wasn't disabled for you until now. But it was intentional, not a mistake.
I haven't noticed any issues with YouTube, but I couldn't tell you what subsample I'm on with my Ultra. That's the only Roku I have connected to a 4K TV.
iPlayer is (I believe) an UK only player, so can't comment on that one.
Dan Nvidia Shield, Roku Stick (3600), Ultra (4640), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.
Thanks for the note here. Sorry to hear about the hiccups you're running into!
As @atc98092 mentioned, the auto-adjust display refresh rate feature no longer applies to the Netflix channel as of OS 9.1
As far as the other issues you're reporting, we'd like to help investigate this further. Could you please send me a private message with your Roku's serial number from Settings>System>About, the specific model of Marantz receiver you are using, your current display resolution selection in 'Settings>Display type', and a specific example of content in a channel for the 4:2:2 subsampling issue you reported? Please also include an example content title for the audio sync issue you are experiencing with the iPlayer channel.
I'll follow up directly from there to continue looking into this.