rdp_sync_on_image freezes session

Hi! I debugging my RDP script, I and find strange thing. When script doing heavy operation, i've see "sync on image" step. But I see in the "Run-Time Viewer" that RDP session freezes when rdp_sync_on_image start performed. The image sync time out is intended for waiting, when some proccess completes. But nothing happened, when sync_on_image performed. If it starts performed when some window opened, the window FREEZES in the middle of the animation! I need use lr_think_time, which the bad way.. How I can avoid freezing, when using sync_on_image?