In other words, when you go back into Lightroom from roundtripping to Photoshop, you're looking at the newly created file.
When going out from LR, it bakes in the metadata edits during export, saves a new file, then automatically imports it into the LR catalog. Thus this new file only has the Import step in its history.
If you need to go back and edit the original raw/dng file (which will still have all its history) you will need to also redo the Photoshop edits.
For this reason its a real good idea to be as sure as you can that you are "done" with your LR edits before roundtripping to Photoshop, especially if you need to perform any significant work there.