Hello,
Not sure if this is a bug, but it looks like Cura doesn't know where to put an STL file when it is opened while a 3MF file is already loaded.
<iframe width="640" height="650" src="https://screencast.autodesk.com/Embed/Timeline/186cc8e3-b4ec-47dd-8752-9f975bef6fea" frameborder="0" allowfullscreen webkitallowfullscreen></iframe>
If the above iframe doesn't work (I don't expect it will and there's no option to embed any videos), the following link is to a screencast of the potential bug:
https://knowledge.autodesk.com/community/screencast/186cc8e3-b4ec-47dd-8752-9f975bef6fea
The first part of the screencast refers to the post linked below - opening a 3MF file places it at the origin contained within the 3MF from the exporting application; STL doesn't contain origin information.
The second part of the screencast from 1:30 demonstrates what could be considered a bug:
When exporting a 3MF file from Fusion 360 and opening in Cura, Cura defaults to place the object at the same origin it was in Fusion 360, i.e. it takes the origin from the 3MF.
Exporting an STL file and opening in Cura always (normally) goes to the centre of the build plate.
When the STL file is already at the centre of the build plate (this isn't 0,0 because that is in the bottom left corner, but it is 0,0,0 in Cura world), opening the 3MF places it correctly at the origin, which is 0,0 (Cura world -110.0011,110,0).
But, when the 3MF is loaded first, the STL file is placed outside the build plate, on the right hand side, at 200,0,0 in Cura world.