i think it must be the issue, upon further futzing over the past few weeks, but i don't understand how to make it not do that.
i did try to redownload, and with some versions, that *does* work, but only for versions of minecraft i've played before this issue began. since i tend to play minecraft in fits and starts that can be months apart, that's 1.21, 1.20.2, and 1.19.2. for others, it displays minecraft as net.minecraft and on trying to redownload the message says "Null jar is specified in the metadata, aborting."