I usually set my D800 to ISO100, f14 and 1/200 for when I'm in the studio. Last week I had a shoot, and I accidentally knocked it 1/250. That should be fine since the D800 can sync at that speed, but I'm going through the photos now and I have a horrible line in some of the photos from the second curtain. It's more obvious in some pictures than others. In a few it's just a slightly dark gradient at the very bottom of the image, but in others there is a pure black line. Is this an issue with my camera, or has Nikon exaggerated the sync speed in the specs? If I wasn't shooting with a white background I might not have noticed it in some images, but in others they are completely black at the bottom.
A7II: 16-35 F4, 55 1.8, 70-200 F4
or try this..
Nikon body "setting" in "FP" or not. and that was my third thought as well.
Yongnuo is a little bit more forgiving and connects to more units, I bought 2 Yongnuo YN622N triggers, put those on High Speed Sync and shot with 1/4000 with the D600 and SB-800's without a problem. They had no problem with the Bowens Gemini 400 with 1/250, you can't go higher because the Bowens cannot do HSS.
Of course the normal flash units should be able to do HSS if you want to use that.
Elinchrom has a studio light for HSS and there is a Fomex G600 on the way for super speeds.