0 negative weights for tWZ LO

Dear experts,

We are working on the implementation of tWZ that only has the particularity that the Z decays hadronically, the process card
is the following:

import model sm-no_b_mass
define p = p b b~
define j = j b b~
define l+ = e+ mu+ ta+
define l- = e- mu- ta-
define vl = ve vm vt
define vl~ = ve~ vm~ vt~
generate p p > t w- z $$ w+ w- /h @0
add process p p > t~ w+ z $$ w+ w- /h @1
output st_tWZ-Zto2Q_5f_LO

The decay is handled in madspin as follows:
define q = u d s c b
define q~ = u~ d~ s~ c~ b~
decay t > w+ b, w+ > all all
decay t~ > w- b~, w- > all all
decay w+ > all all
decay w- > all all
decay z > q q~
# running the actual code
launch

And the PDF that we are using is NNPDF31_nnlo_as_0118_mc_hessian_pdfas (lhaid: 325300). So, as far as I understand, in setups in which LO calculations are mixed with NNLO PDFs, one can obtain negative weights even at LO. However, after producing some modest amount of events (~2k events) in this setup (and actually, after showering), the report says that the negative weight fraction is exactly 0, which sounds very strange to me.

So my question is: should we worry that there are 0 negative weights in this process? Or is this something that might happen in some corner cases in which the point that I make above does not apply?

Feel free to correct me in anything I may have misunderstood on how this works, and thank you very much in advance!

Best regards,
Carlos Vico