Purchased songs keep disappearing from iMac

My iMac (27-inch, Late 2013) is running macOS 10.15.7 Catalina. It cannot be upgraded beyond Catalina. Songs which I purchased from the iTunes store in 2020 to date keep disappearing from Music. I downloaded the songs, then 2 - 3 days later they disappeared from my iMac and had to download them again. The songs keep disappearing and I have to keep downloading them. Even the playlist with those purchased songs disappears. This happens only with songs which I purchased from iTunes from 2020 to date; all songs purchased from iTunes before 2020 are fine. I called Apple support on 2 different occasions and no one was able to help. Any suggestions?