Cross-staff glissando with crescendo and dynamics

[David Santamauro]

Hi, I'm not understanding the warnings I'm getting with the following snippet. 
I've included a possible work-around that is musically identical, but visually 
unappealing:

%%%%%%
\version "2.22.2"

\include "english.ly"

\score {
  \new PianoStaff \with { instrumentName = "Harp" } <<
    \new Staff = "s_harp_upper" {
      \relative c'' {
        \clef "treble"
        s1        |
        s4 r4 r2  |

        % work around without warnings
        R1*2
      }
    }
    \new Staff = "s_harp_lower" {
      \relative c {
        \clef "bass"

        f,1\p\glissando\<
        \change Staff = "s_harp_upper"
        bf'''4\!\mp
        \change Staff = "s_harp_lower"
        r4 r2

        % work around without warnings
        f,,,1\p\glissando\< \clef "treble" bf'''4\!\mp   r4 r2
      }
    }
  >>
}
%%%%%%

For the cross-staff I get 4 of these:

programming error: My pure_y_common is a VerticalAlignment, which might contain 
several staves.
continuing, cross fingers

Can someone explain what those warnings mean and how to code the cross-staff 
gliss with dynamics and crescendo. correctly?

Thanks