I'm new to the forum, a little bit of presentation: I work in a design office under Pro/E - wildfire 5.0
When I import .stp files, I have the choice to open in .prt or .asm
In most cases, if I want to open a .stp as a part (.prt) but which is originally an assembly, the software opens it but not correctly, all the parts are piled up on the same point...
It may seem logical from a certain point of view: you ask for the opening of a room so everything is brought back to the origin. To work around, try opening in asm mode and then convert the asm to prt (via "save as" or "export", in general).