There appears to be an off-by-one bug in the code that decides if the disassembly window needs to be scrolled / repositioned. If the next statement highlight reaches the bottom of the window it can completely disappear for one step before it is repositioned at the top.
The best work-around is to size the window so that the last line is only about 90% visible, which is generally good enough, and save your settings.
Version: WinVice 2.4
O/S: Windows XP
Should be fixed so that the next statement is always made fully visible after stepping.