This is happening to me on my Shield. No idea why.
It's a bug. It happens under Shield and Mi Box.
When an app supports cast features and control like when you play a video and remote controls show up on your phone then the volume goes to 50%. It happens on Netflix, YouTube, Spotify and on almost all apps support cast capabilities and send to Google what you watch and can controlled by Home app.
It's very annoying.
It's a bug I've been having on my Formuler GTV for weeks, it happens mostly when I open or play a video on YouTube.
One easy way to fix your problem is to call your delear or visio customer service center : ask the combo key to access to hotel mode setting. All hotel mode allow to set the default and max volume on switch off switch on.
I posted about this a few weeks ago because it suddenly kept happening to me with the Chromecast with Google TV and it drove me crazy. I always figured that it was something got corrupted on my device because no one else was reporting this. I contacted Google who basically thought I was crazy, and walked me through the steps on resetting the device and remote and all that didn’t help. Eventually they ended up sending me a replacement device and it has happened a lot less. Because I have the CCwGTV set to let me control the volume with the tv, I’d get a “Use a remote to adjust the volume. See Chromecast settings on this TV for more options.” message. https://imgurupload.org/files/FAE03496-74E3-4C55-84F5-6FE270CDA9BE.jpeg When I’d set the device to allow the remote to control its volume instead, I’d see the volume drop to 50%.
Ok now I know that Google put out an update to the web receiver (what is used to display the content to the device) on March 30th and it specifically mentions it fixed a bug in Chromecast with Google TV where the media controls overlay was displayed when casting photo content to the default receiver. That “media controls overlay” sounds familiar, right? https://developers.google.com/cast/docs/release-notes
Everything worked fine for the first two months and then it started happening in Hulu and YouTube and HBO Max. It was to the point that I was just gonna stop using my device. I’ve everything under the sun. Factory reset the remote and re-downloaded the firmware, re-paired the remote, did multiple factory resets of the CC, made deals with God, uninstalled app updates, cleared the cache of all the apps, poked around the developer settings, and nothing worked except replacing the device. Got the replacement March 21st and set it all up and everything worked fine for the first four days, and then I go to open the YouTube app and boom, the message pops up and my stomach sinks. Knock on wood it hasn’t again since. I was literally sick over this because of the anxiety it was causing me.
This was discussed in this thread as well. https://www.reddit.com/r/AndroidTV/comments/mkb1gl/comment/gtmis97?context=3
Also happening to me on my Mi TV Stick whenever I play a video on YouTube.