Based on a build off of r260522: Log onto beta.music.apple.com. On one of the albums select (...) The context menu that appears is missing the backdrop filter. Toggle it off/on. It appears.
Created attachment 397283 [details] Screenshot
<rdar://problem/62230339>
This requires dark mode to reproduce. In light mode, the menu is solid and doesn't have a filter behind it at all.
@Jon Lee I assume there was a change on beta.music.apple.com, I can't reproduce this in dark mode right now. Do you have any details what filters were applied and what the zindex of the element was? I think I have a similar issue and a potential fix.