Wow, the coil package is not just moved, it's rotated 90 degrees.. look at the coil former there.
From the lessons learned with the MELF packages (parts where separated entities) I build any model solid in Inventor (extrusions etc. have overlap), so there shouldn't be any possibility of parts to become loose.
I can confirm that FreeCAD sees two pieces there though - will investigate.
I found that a mirror operation that creates the copper-coil inset on the 'other' side of the coil package is causing this, if I suppress it and export a step model and import it into FreeCAD it's just one object.
I can solve that, no problem.
I can't explain why it separates like that though, as the creation of the coil package (with those separation planes) happens earlier and should be unrelated..
Somehow in your tool-chain something is applying moves/rotations to the 1st piece only and leaves the 2nd piece where it was.. can you nail down or guess which step might cause that?
I should be able to avoid several-parts-per-model thing.. we stumbled across this with the MELF I think. Can you check with StepUP if this is a potential problem with above transformer please? Thanks!
I changed the modelling slightly and found what might had been the problem.. a projection sketch profile of the coil former had been used in the extrusion feature that creates the copper inset. Mirroring that causes the problem. I'll have to repopulate the parametric tables with the different values for the series again to export every model for this change to take effect in the STEP/VRML files - will see if I can automate this a bit more, might be useful as one can see