wrong modification assignement

[Umbs]

Hallo,
we are having a problem with a custom modification we have add to a
Scaffold experiment.
The experiment consists in the in vitro acetylation of a protein,
using an acetyl-CoA labelled with to two 13C (Acetyl 13C(2),
monoisotopic mass delta: 44.017 Da, modified residue: K).

Using Mascot, we have correctly assigned the modification, then we
have tried to use Scaffold and X!tandem, adding the custom variable
modification, in addition to methionine oxydation and N-term
acetylation. The problem is that in the results the software adds two
additional modificatons (carboxyl on K, and N-carboxyl on n, both with
a mass shift of 44 Da), while the Acetyl 13C(2) becomes located on X
residue. Moreover, the peptides identified as 13C-acetylated in Mascot
becomes carboxylated, only.

Could you please help us in solving this problem?

Thank you very much in advance!!

Umbe