Cannot reproduce this, was it happening with a particular FOV option? Also, did you close the menu by pressing the Esc key or by selecting the ‘Return to Game’ option?
Was your character briefly stuck with the falling animation?
Sounds like you’re experiencing some rubber banding. Follow the instructions here to report the information:
What items are you having trouble smart stacking? Can you turn on the debug information and check the colour index for the items in question?
It could possibly be that the world might have been down. When this happens, could you turn on the debug information and check for any errors?