stem length and change staff

[Tiresia GIUNO]

Dear all,

in the attached files (.ly & .png) the alteration of the first b' in the
second bar produces an incorrect length of the stems and an incorrect
position of the beam.

This behavior does not appear in the third bar, where the third chord
is modified. Is there any reason for that?

To correct this situation, I'm overriding the beam's position with
        \once \override Beam #'positions = #'(-4.5 . -5.5)

Any other suggestion?

Thanks a lot,
T

changeStaffStem-example.ly
Description: Text Data

changeStaffStem-example.png
Description: PNG image