Selfdrive,
Yes, is the simple answer. I get the same effect when checking with Previewer 2, but all is well on K4PC, K3 Device, and KDP previewer. That's with everything, image wise, set to exactly 600x800 px.
Actual device testing is the only real absolute way to be 100% sure, and K4PC to be 99% sure.
If it looks ok on a K3/4 reader it will sail through the KDP conversion and come out exactly as you see it on the device. Usually
That's why most folk are now uploading mobi only.
Kindlegen & Previwer work, but not really that well to be honest, at least compared to alternative options. Although, they are fairly accurate for layout checking.
Best of Luck