I also have a new suffix that appears in my design tree when I show component descriptions in the tree display.
This came up when I updated from 2017 to 2018 about two weeks ago. I did not attend my local VAR's whats new in 2018 meeting, but read through the whats new in 2018 pdf and did not see this. It looks to be a part instance indicator that populates when you have more than one of the same part/asm in your design tree as it matches the <X> instance suffix that has been around for a few releases. That would explain why you do not see it on every part/asm in your design tree.
Not sure why this comes up when displaying descriptions in the design tree, but can be confusing when using dashes in your part numbering/file naming.
I would also like to know if these can be removed or toggled off.
Seems counter productive to have this "-X" suffix added into the new 2018 release when the exact same instance call out comes in with the "<X>" behind the file name in the Design Tree.
I hope someone else may provide some more information on what this new suffix is intended for or if this is some kind of bug with 2018.
I just tried it on an old computer with SW2015 and the suffixes weren't there. I strongly suspect this is a bug.