I had a major misfire issues with a Flashpoint Zoom Li-on III (Godox V860III) and a Z7II. One of the troubleshooting tips with the Adorama rep was to set the shutter to mechanical (instead of electronic front curtain) on the Z7. It didn't fix the issue and I got the flash warrantied. I have other Flashpoint (Godox) flashes that I've used with the Z8 and cannot go higher than 1/200 shutter speed without a black line appearing. I also have a new Z6III and keep the shutter at mechanical whenever I'm using the flash.
So, there may be an issue with the V860III and electronic shutters, or in your specific case, I would try setting the flash to "normal" instead of High-Speed Sync and set the Z9 Custom Settings: e1: Flash Sync Speed to 1/200 and see if that eliminates the issue.