I figured it was doing this because I was running a version of RetroArch that was probably two years old at this point, but I just updated to the latest x64 build and it’s still giving me this error whenever I try and view the list of available cores to download.
Updating other things seems to work. Updating cheats works, the core info files seem to be updating just fine, etc. But the actual list of cores seems to be broken from within RetroArch.