I have re-claimed four ghost shells from my collection, and every time it has Mercury specific properties (and elemental analysers). My husband claimed one today and it was also Mercury specific.
We suspect it's a bug.
Edit: last night, I got a shell from the level-up engram - mercury. Then I bought a shell with brightdust from Tess (previously not unlocked) - again, mercury.
I have Forsaken, but not the annual pass.