GNU is really its own thing and not reallyPOSIX anymore. So GNU is right even if they are wrong.

This is not me advocating for GNU. I use BSD utils myself.

On this issue, your were right in a way. My understanding is that the uutils version of dd was respecting the fullblock parameter, causing problems on slow pipes. GNU ignore this and was doing partial writes. Uutils has been modified to match GNU and is “working” now. At least, a tested patch has been submitted.