Help & troubleshooting for channels on your Roku device, including adding/removing channels, logging in to, authenticating, or activating a channel, channel-specific playback issues, assistance contacting channel publishers to report issues, and adjusting channel-specific settings.
You can check out the page for further info, but there is info that may be helpful.
"Supported 4K devices You can watch in 4K on the following devices:
Amazon Fire TV Stick 4K and Fire TV Cube Android TVs, including: AT&T Streaming Box Apple TV 4K Google Chromecast Ultra and Chromecast with Google TV Roku Ultra 4800x, Roku 4k TVs, Roku Premiere, and Roku Streaming Stick+ Xfinity X1 (Xi6) and Flex We’re working to add more devices and 4K movies in the future.
Note Dolby Atmos is not available with Chromecast with Google TV, Roku Premiere, and Roku Streaming Stick+."
and for additional tips (bold added by me):
"Step 4: Try another cable or HDMI port If you're using a streaming player (e.g. Apple TV):
Make sure you're using a High-Speed 4K HDMI cable. Make sure your HDMI cable is plugged into a HDCP 2.2 compatible HDMI port on your TV or audio/video receiver. HDCP 2.2 may not be supported by all of the HDMI ports on your TV (refer to your owner's manual to find out which HDMI port is HDCP 2.2 compatible). Bypass your audio video receiver, HDMI switch, or sound bar, by connecting your streaming device directly to your TV. If you're getting an HDCP error, try the TV steps in this article: I'm getting an HDCP error."
Not sure if these tips help anyone but I figured it'd be worth a try. Since no one is actually 'supporting' HBO Max on Roku, we'll have to figure out the issues ourselves.
Re: HBO Max - some info from their support site...
FYI, this HBO Max support article is inaccurate and out of date (since HBO Max was released for Roku last December) as it does not now nor has it ever had accurate Roku model support information for 4K/HDR/Atmos.