Not sure if this is a bug or not but when you import a mesh via the new Mesh Import feature and then go to drag & drop an STL into the same model the scales end up out of whack, even if it is the same file...I've attached screen-grab for reference and as you can see there is a difference in scale by a factor of about 10...
I found this when I tried to insert my files from within my Fusion 360 cloud project browser after having used the import mesh feature in the new mesh work space.....really should allow drag and drop, that was one of the benefits of loading files up to the cloud in the first place rather than keeping them local.