Direct play constantly buffering on Chromecast with Google TV since July update

I’m having issues with constant buffering while direct playing high bitrate videos on my Chromecast with Google TV. This has only started happening in the last 3 weeks:

  • The stream buffers for 3-5 seconds out of every 3-5 minutes of streaming
  • Began seeing stuttering/buffering issues in July 2021
  • Only happens when direct playing high bitrate 4K HDR HEVC content: video ~50 Mbps will stream fine, but video 60-90+ Mbps will buffer constantly
  • I haven’t had issues with high bitrate content on my setup before. I’ve been able to stream 120 Mbps content in the past with no issues. I've had this Plex setup since February.

Some details about my setup:

  • Plex for Android TV v8.21.1.27073
  • Chromecast with Google TV
  • Plex Media Server for Windows 10 v1.24.0.4930-ab6e1a058
  • LG OLED CX
  • 4K HDR HEVC Remux content
    • Dolby Vision profile 7 (attempted both FEL and MEL)
    • Dolby Audio AC-3 5.1 (and DCA 5.1)
    • MKV container
  • Direct play on my home network. I am the only user on my network.
  • Server is connected via ethernet; Chromecast is on 5 Ghz network using 80 MHz channel
  • Network speed is not the issue. My Chromecast is getting 300-400 Mbps download speeds with other applications

The interesting bit: I have no issue streaming the same exact high bitrate movies to my Android Plex client on my Pixel 3. It only seems to be happening for my Chromecast. I'm able to get 90+ Mbps streaming with direct play. From what I understand, this is the same client (v8.21.1.27073). Is there a reason why I'd have no issue with one device, and buffering on the other?

For example, when direct playing a movie that averages 67.6 Mbps on my Chromecast, after output of my server is ~60 Mbps and it buffers constantly. The same when I am direct playing a movie averaging 89 Mbps: output speeds from my server is ~60 Mbps. When doing the same two movies on direct play to my Pixel 3, the server outputs 60-70 Mbps and 80-90 Mbps, respectively and the stream plays smoothly and flawlessly.

The only thing I can think of is if the network cache on the Plex app is not big enough to support fluctuations in the bitrate or of my network connection.

Please let me know what I can try. I can provide log files if necessary.

edit: Things I've already attempted:

  • power cycling Chromecast, Windows server, and router
  • uninstall/reinstall Plex on Chromecast
  • factory reset and reinstall Plex on Chromecast
  • uninstall/reinstall (and rebuild libraries) Plex Server on Windows
  • tried both 5 GHz and 2.4 GHz for my wifi