You can burn the beam while it's in the niches, but it won't disappear

[eriktorbjorn]

>LOOK
Narrow Passageway
You have reached the west end of the passage. Before you is a door surrounded by
a heavy timber frame. The door is entirely blocked up with plaster. Toward the
bottom of the north wall and the south wall, about three inches off the floor,
are two small niches. There are some hieroglyphs on the plaster.
Stretching across the floor from niche to niche is the wooden beam. You are
standing on the beam.

>BURN BEAM WITH TORCH
The wooden beam catches fire and is consumed.

>LOOK
Narrow Passageway
You have reached the west end of the passage. Before you is a door surrounded by
a heavy timber frame. The door is entirely blocked up with plaster. Toward the
bottom of the north wall and the south wall, about three inches off the floor,
are two small niches. There are some hieroglyphs on the plaster.
Stretching across the floor from niche to niche is the wooden beam. You are
standing on the beam.

That's because while V-BURN specifically checks if you're burning the beam, it only handles the case in this room if you've removed the plaster:

                  <COND (<AND ,ON-BEAM
                      <EQUAL? ,HERE ,WEST-END-OF-PASSAGE>
                      <NOT <IN? ,PLASTER ,HERE>>>
                     <JIGS-UP ,AIR-WALK>)

[eriktorbjorn]

The bug happens whether or not you're standing on the beam. This is just how I first discovered it.