This took me about three days to figure out & I wanted to make a note on the off chance it affects someone else.
I was using this program as the basis of a system to generate a bunch of SVGs using React & saving them to a CAR for download. This program includes the class MapBlockStore. That class includes the method put ({ cid, bytes }).
In more recent editions of ipfs-unixfs-importer, the expected blockstore put signature is put (cid, bytes) (no wrapping object).