My previous experience with ghost images (PF Core + LX FlarmView) was when a gliders signal was lost. The combo would leave the traffic displayed for a set time period for general awareness. You could cycle to the target to see the CN but not get any altitude data. As it came back in range, the target filled in again and presented normally. As you were thermalling, perhaps you were experiencing a similar thing as the fuselage blocked your signal? I note your question refers to PF Portable thus YMMV.

CJ