Reproduced with given files and resolved as https://www.ventismedia.com/mantis/view.php?id=15407

Could you send me some tracks like B,C,D? You can use some file sharing service like Dropbox, OneDrive, etc. and send me link by private message. Thanks.

Yeah, this occurs in the last few builds that I'm using MM5.

I've tested and think it's related to tracks. Suppose tracks A, B, C and D.
Track A is good and both small and big pictures are showing correctly when you play it. And when you change track from/to it.
Track B, C, D are not good. The pictures are showing correctly when you change track from A to one of them (Or vice versa). But not if you change from/to B/C/D.

I have lots of tracks like B, C, D. So now?

Do you use last beta version (build 2152)? If so, please try to notice, when this happen, if this is track/album related, if you skip to next track manually, or some other context. I was trying to reproduce it, but for me it never happened. It used to in the past, but not in the last builds.

Thanks but you are misunderstood my issue. It's not about the bottom right window that has 2 modes. It's about the Playing node of the media tree. There are 2 small and big pictures showing there. I don't know why but sometime the small picture doesn't change when the playing song changes.

Be sure that you have the mode "Playing" and not "Selected" in the small image window.
You can just click "Selected" in the header to toggle it to "Playing"
Or click the three dots overflow menu and select Mode > Playing

There are 2 small and big images showing in playing view but sometimes the small image isn't related to the now playing song. Although I switch to another song but the small image doesn't change accordingly.

h-ttps://i.imgur.c-om/uPpMpYd.jpg