Sounds like an issue with that copy. I don’t notice an IQ hit at all in my copy when activating the 1.4x.
I know some IQ degradation happens, such is the case with any TC, but it’s not noticeable or worth worrying about with 45MP and typical post processing.
Nikon adjusted it and it came back a bit better but still not as good as a bare 400 with 1.4x. It was more like the Canon 200-400 1.4x, and long term I'd be nervous if it can lose IQ and need servicing.
