I also have that problem! I've encountered it in every version of WinUAE I've used. I kept meaning to post about it before but always forgot.
Misc info:
* does not occur on my real Amiga 2000 (+ 030/882)
* occured in pre-JIT versions of WinUAE
* occurs in both Imagine 3 and 4
* occurs only in the FPU versions of Imagine
* occurs under all CPU and chipset settings
* occurs under both kickstart/Workbench 2.0 and 3.1
I've attached a couple rendered pics that illustrate the problem...