My plan to retrieve Piper has hit a snag. My goal was to reach the mission "In Sheep's Clothing" where Piper was supposed to reappear. The thing I didn't know until recently is: That mission doesn't trigger unless I have finished the main storyline, or, I have taken actions to get myself excommunicated from a particular one of the three main factions.

Neither appeals to me. The whole point of having Piper would be to have her travel with me as I complete the story missions, so waiting until the main story is done before getting her back wouldn't achieve my goals. And getting myself deliberately excommunicated from this one faction earlier than normal will close off all of their remaining faction-related missions. (Plus, being a member of their faction has some benefits that I don't want to lose.)

My plan is now to do as many of those faction-related missions as I can until I reach the point of no return with that faction, then get myself excommunicated, then go get Piper, then finish up the rest of the story missions and side missions with Piper as my companion.

Bleh.

Bad bug, Bethesda.

(PS: I'm on PS4, so I can't find nor fix Piper with console commands.)