YouTube Screencast from Android to TV gets interrupted when viewing MP4 video on Phone

When I use the built-in Screencast feature on my Android phone (Oppo Reno 4), I am able to cast a YouTube video from phone to my TV ([LG] webOS TV LM630BPTB), and then after I swipe down and tap Screencast options and choose *Cast "YouTube" only*, I am able to even use my phone at the same time while it is casting to the TV. Perfect. So now the video and the audio are all forwarded to the TV. The thing is, theoretically I should also be able to view a simple MP4 file on the phone simultaneously and also hear audio locally on the phone if some advanced programming techniques had been employed with suitable hardware. But unfortunately, when I try viewing an MP4 file on my phone or scroll down on FB and suddenly encounter a video in my feeds, the screencast gets interrupted. Is there a solution to this problem? What is the solution? It seems like there is no simple solution to this problem within the Android operating system itself. The screencasting feature probably is not designed to handle multiple video streams at once, and the limitation is more of a hardware and software design constraint. Anything I can do about this? Maybe install a certain app?