Actually, I've had a weird moment with "official" batteries as well. Ordered my Z6 from Panamoz and at the same time ordered a spare EN-EL15b from them as well. All arrived in the same box, but whilst the one "inside" the accessories box of the Z6 worked fine, the "spare" one I'd ordered worked a little weird. Whilst it would charge in the supplied AC charger, it wouldn't charge via USB like the other one, and the "charge" LED on the camera would blink furiously.
I noticed that the "original" battery with the Z6 has no "EN-EL15b" inscribed on the top surface (as per the photo above), but the "spare" one did. I contacted Panamoz, and as usual they were excellent and apologised for the inconvenience, and asked me to purchase one locally (which I did from Clifton cameras), send them the bill and they would reimburse me (which they did in a day). When the new battery arrived from Clifton Cameras, I noticed it was like the "original" one supplied with the camera i.e. no model type (EN-EL15b) engraved on the upper surface of the battery. This one also charges beautifully on the Z6 as well over USB.
Therefore I can only conclude from my 3 samples (2 without the engraving which work as expected, and one with the engraving that doesn't charge via USB), that the ones with the engraving are the fakes - and I guess there must be a load of them about if it's now affecting 2 suppliers.
As a final point of note, whilst the "fake" battery doesn't charge via USB, it does still seem to work OK in the camera (if charged via the AC charger), and still shows battery life, shots taken etc. in the camera, but I've relegated this unit to the role of emergency 4th spare after my other two genuine units, and a non "B" version original unit I had left over from my D500.