[Forewarning: This bug can’t be recreated at will, but I’m seeing it often enough that I thought it deserved to be posted.]
Here’s the behaviour I’m seeing, and it appears to be new to Mockups 3 as far as I can tell. I will create a symbol from some inserted and grouped elements with sub-groups as well, and then I will place a second instance of the symbol into the screen. So far so good. Then I will open the second symbol instance for editing, so I can move one of the elements of the symbol to a new location. As I try to translate the element, it will immediately snap back to the original location. This is true if I try to drag it, or if I edit the position in the property inspector.
Typically the only solution I can find is to delete the symbol and all instances, and start again. (hence the can’t reproduce every time.)