After commenting out random things, it turns out specifying `type: "bytes"` in the ReadableStream underlying object was the problem. None of the things Cursor said even remotely hinted this was the issue.
Tried out Cursor to have it help solve an Cross-Origin-Embedder-Policy error i've been getting on Safari when an iframe is trying to access a Service Worker URL.
It was just a series of "try this" and "try that". None of it worked. Complete waste of time.