@User ngspice 33 is out, according to release notes model binning is fixed:

+ Use W/nf for selecting binning model.
    + Add wnflag as an instance parameter.

Still unsure if a

.option wnflag=1

must be given or is assumed as default ... will test out. By RTFM (ngspice-33-manual.pdf) I got this:

If the variable (17.7) ngbehavior is set to spe with the command
set ngbehavior=spe
is set in .spiceinit Spectre compatibility mode is enabled. True compatibility today is still
far away. The only action available for now is the use of the MOS device instance parameter
nf . If nf is given and larger than 1 and Spectre (or HSPICE) compatibility is enabled, nf is
used as a divisor to the transistor width W given on the instance line. The resulting W/nf is now
used to select the suitable device model in the binning process. This procedure is of interest for
a multi-gate transistor, which has a total width of W , but each finger is model according to the
model given for W/nf .

.