I have not had the experience of Align/Move pulling pizz and arco below the staff, but then I usually am using the Windows version of Finale, although I would find it surprising if that made a difference.

I did notice that the plug-in moved the pizz and mute (but not the arco and open) closer to teh staff as the dynamics below moved downward/farther. But pizz never moved below the staff.

Nonetheless, the unpredictability (between pizz and arco, mute and open) is a flaw, and probably the inability to include or omit above-staff expressions should be added to the checkboxes in the panel. How much this involves going back to Tobias to change his plug-in I do not know.

The behavior you described is what I was talking about.  I didn't mean to imply that it moves all the way down below the notes.  It seems to effect pizz, but not arco, and you mentioned "mute".  I wonder how many more "over the staff" expressions move when "Align/Move is used?  I was just checking over an orchestra score I recently wrote and noticed several "pizz" marks colliding with objects, and realized that that must have happened after using the Align/Move plugin.  

Have you tried moving the arco and pizz to a different expression category, or changing their positioning data to manual positioning? I don't know if that would help, but it's worth a try.

I have encountered this numerous times on Windows. I don't think it's a bug because the categories you are able to check in the plug-in are broad; it's just "text expressions," "hairpins," and "shape expressions." Naturally, "text expressions" would include any and all categories that are define by text. The exception might be the default "score list 1" which places the expression on the "top staff."

MakeMusic staff voiced that they would be including an "Engraver Hairpin" similar to "Engraver Slurs." Hopefully this will include positioning/aligning hairpins on a baseline.