Redefining stencils in 'tweaks causes grobs to be printed twice

[Valentin Villenave]

Greetings everybody,
I'm not quite sure if this qualifies as a bug, but I can't figure out
what on Earth is causing this strange behavior (maybe the
tweak_engraver?):

% Changing the stencil expression through the
% 'tweaks alist causes a grob to be printed twice.

fzz =
#(let ((m (make-music
        'AbsoluteDynamicEvent
        'tweaks (list (cons
            'stencil (lambda (grob)
              (grob-interpret-markup grob "blah")))))))
      m)

\relative c' {
  \once \override DynamicText #'stencil =
  #(lambda (grob) (grob-interpret-markup grob "blah"))
  c1\fz
  c
  c\fzz
  c
}

Cheers,
Valentin.