Hello,

chapter XXI, section 93 of the manual says about \keys_bool_set:NN:

"Creates code to set ⟨bool⟩ when ⟨key⟩ is given, with setting using ⟨scope⟩ (l or g for local
or global, respectively). ⟨bool⟩ should be a LATEX3 boolean variable."

First of all, I think the scope description is incorrect since the scope is simply used to construct control sequences like \bool_set... and \bool_gset..., but there is not \bool_lset...
The second argument of \keys_bool_set:NN should thus be empty if the assignment is to be local, in which case the argument specifier should be "n", not "N".

Then line 1320 of l3keys.dtx has

\keys_property_new_arg:nn { .bool_gset:N } {
  \keys_bool_set:NN #1 n
}

Note the scope "n" instead of "g" (or rather "{g}"), which leads to an error ("Missing \begin{document}").