[Rivet] Missing of Matrix Element particles with status 23 cut

[Yichen]

Dear Pythia8/Rivet experts,

I'm using Rivet to analyze a MG5+Pythia8 ttbar+photon sample.
And I'm trying to get the matrix element particles using status==23 cut.
But it turns out that in many cases, "status==23" can't capture all the 
matrix element particles!
(I did similar study for Pythia6, where all matrix element particles can 
be identified with status==3)

Do you know what could be the source of the problem?
I'm not sure if it's due to Pythia8 or Rivet. So I send this email to 
both mailing lists.
And please let me know if you need more information!

Thanks a lot!

Yichen Li