I just tried that, and also addes some screenshots to make my "problem" more clear.
I just added an album to my Poweramp folder that I for sure never added before to that specific phone (Brother Bear); I am using my OnePlus since last week only.
The album only contains of two songs. First, I copied it to my storage through the USB cable. After a library rescan (normal, not full), the songs show up on top of "All Titles" if I sort the list by "date added to library", but they won't show up in "Recently Added" at all. If I rename one of the songs and do a normal rescan again, nothing changes. I then deleted the album and copied just one song through Dropbox into my Poweramp folder. And, as you can see, that song now shows up in "Recently Added". So the problem must be somewhat related to my cable/MTP method? Which also is strange, because if I do the same on my older Sony, everything works fine and the songs show up in "Recently Added" even through the cable method.
"All Titles" after copying through USB cable:
"Recently Added" after copying through USB cable:
"Recently Added" after copying through Dropbox:
Edit: I just now bought a brand new track of one of my favourite bands. I copied it to my phone through CABLE, and after rescanning, it DID show up in "Recently Added".
So, my theory is that there has to be a difference between the to MTP versions of my two phones. Only older albums and songs that are transferred through USB connection are affected by the bug.
It would be super interesting to know how the "Recently Added" playlist exactly works under the hood.
Edit2: I think I found the cause for my problem.
When copying files to my old Sony phone via USB cable, the "last edited" timestamps of all the mp3 files get changed to the exact timing the copying process is done.
When copying files to my new OnePlus via USB cable, the "last edited" timestamps do NOT get affected.
I now assume that the MAIN condition for tracks to be added to the "Recently Added" playlist is that they were EDITED lately, not added to the library?
That would explain why new songs show up in the playlist, while (from a creation/editing perspective) old songs don't.
Seems like there was a change in the MTP protocoll between Android 8 and 11, or between Sony and OnePlus.