import an stl file, print it a few times (can't say how often as i've been doing batches of 10-15 at a time)
exit preview, import another stl file, deselect previous stl file but don't delete it
enter preview mode
both STL files show up in gcode and try to print.
only way to fix is to go back and delete the original stl file.
verison
3.0
and 3.01
windows x64
monoprice 3dp