I think there's an issue with the pipeTo() algorithm that prevents writing all the data. When the underlying source calls close() the state of the ReadableStream changes immediately to "CLOSED". If there is a read() on the readable ongoing at that point then pipeTo shutdown starts immediately, prohibiting the data that was read from being written to the WritableStream.

I'm not 100% certain that this is a spec issue yet but I am making a note of it to look at it again later. I will close this bug if I conclude there is no actual issue.