I'd report that to Lee in the U77 release candidate thread. Here's your snippit with a small tweak to help show that the mousex and mousey functions are returning the wrong coordinates when used in combination with position mouse.
make object cube 1, 1
do
yrotate object 1, wrapvalue( object angle y( 1 ) + 1 )
if mouseclick()=1 then position mouse screen width()/2, screen height()/2
text 0, 0, str$(mousex())+":"+str$(mousey())
text 0, 10, "should be:"
text 0, 20, str$(screen width()/2)+":"+str$(screen height()/2)
loop
Edit: Actually, I think the mousex and mousey functions return the desktop screen coordinates of the mouse when using the position mouse command. Whether or not it's supposed to do that I don't know.
A clever person solves a problem, a wise person avoids it - Albert Einstein