I guess I've discovered the error.
This is the sequence of statements in "update178.gmic"
#@gimp Frame [fuzzy] : gimp_frame_fuzzy, gimp_frame_fuzzy(0)
#@gimp : Horizontal size (%) = float(5,0,100)
#@gimp : Vertical size (%) = float(5,0,100)
#@gimp : Fuzzyness = float(10,0,40)
#@gimp : Smoothness = float(1,0,5)
#@gimp : Color = color(255,255,255,255)
#@gimp : Sep = separator(), note = note("<small>Author: <i>David Tschumperlé</i>. Latest update: <i>12/29/2010</i>.</small>")
gimp_frame_fuzzy :
-repeat $! -l[$>]
sx={$1%*max(w,h)/2} sy={$2%*max(w,h)/2}
-frame_fuzzy $sx,$sy,${3-7}
-endl -done
If you change the beforelast statement from
-frame_fuzzy $sx,$sy,${3-7}
to
-frame_fuzzy $sx,$sy,${3-8}
the transparency is kept (it's the parameter 8).
Could you double check David and fix it? Thanks