Rule in Question is covered on pages 167-168
Make a PSR (with modifiers). A failed roll means take damage as if the attack came from the front or back (depending on movement) or Left/right side for Lateral Movement of 4-Legged Mechs.
Under no circumstance does the failed roll mean you fall.
Now, here's the Kicker (no pun), Even though your 2-Level tall mech enters a Level 1 Building, the mech is then for all intents and purposes
while inside the building hex only 1 level tall, for movement, etc, but 2 levels tall for LoS etc...weird I know.
Page 167:
BUILDING LEVELS
Treat building levels exactly like other terrain levels for line
of sight and movement, with each level of a building measuring
roughly six meters high. These levels do not represent a
specific number of floors, but rather are an abstraction of the
building’s interior.
This means that even though a ’Mech stands two levels tall,
when inside a building hex it only occupies a single level at
a time. However, it still is considered to rise two levels above
the underlying level it occupies in a building hex for purposes
of LOS to any hex that is not a non-adjacent building hex. For
more information, see Changing Levels in a Building, p. 169.
What does this mean? Your Level 2 Mech, entering a level 1 Building can take damage to it's head.