The NNS App isn’t using dfx’s dynamic asset canister, but rather has the assets hard-coded in it’s single backend canister, with it’s own implementation of the http_request interface. And as you can see there, it doesn’t even know about streaming:
So how does it handle assets larger than 2MB (if it even does)?
Turns out that the maximum response size has been increased multiple times since the asset canister has been created (mostly in response to the NNS canisters themselves growing larger than 2MB, I believe, given that there is no way to send larger request to canister_install). But nobody told the asset canister about the increased limit, and there is no dynamic discovery of that limit, so the asset canister still tries to stream them, which then confused the service worker.