I recall you had a previous thread on this issue ? When you launch just Stucture, does it launch on the same AutoCAD Version / Vertical as Plant ?

I once had a similar issue with Equipment not loading into my Autocad session only to discover I had at one time run the SetupProfile exe and Equipment was still set to launch on BricsCAD.

My best guess is something similar is happening on your end, possibly Plant on 2018 Autocad / Structure on 2017.