I have a 4K TCL Android TV and a Galaxy A52 phone. So far I didn't experience any problems when I mirrored my phone to TV via Google Home Chromecast. However, for last months, I cannot watch a video without my fullscreen rotated phone getting horribly arranged on my TV. When I cast my phone to TV after playing a video on a website (desktop mode and fullscreen mode).
Massive black bars everywhere and the aspect ratio is horrible it's like I'm watching on a tablet. I've adjusted my TV's aspect ratio and Chromecast built-in settings, my phone included. However, I could not fix the problem. Please someone help me I'm gonna go nuts.
When I cast to my android tv, it usally when play a local file. So I'm either using mx player or x player or km player. Depending on what I require, subtitles etc. You can change the aspect ratio on these video players. You have the ability to set your own on km player. On mx player I usually just use the stretch option. It fills the whole screen on my phone, which fills the whole screen on my TV. Hope that helps. Playing video from websites still give the black bars because I have little control over it.