Psi not displayed in legend

[Wolfgang Engelmann]

I have a greek Psi in the text, the same Psi in the legend of a figure.
  The former is displayed in the pdf output, the latter not.

  The error message is:
  LaTeX Font Info:    Font shape `TU/LibertinusSerif(0)/m/it' will be
  (Font)              scaled to size 10.95pt on input line 253.
  LaTeX Font Info:    Font shape `TU/LibertinusSerif(0)/b/it' will be
  (Font)              scaled to size 10.95pt on input line 253.
  Missing character: There is no 𝜓 (U+1D713) in font
  [LibertinusSerif-Regular.o
  tf]:mode=base;script=latn;language=dflt;+tnum;+lnum;!

  I guess, it has to do with the emphasized text in the legend, which I 
have set. Is there a way to get the missing Psi?

  I use Lyx Version 2.4.0-beta2 (Sonntag, 11. Dezember 2022)
  Qt-Version (Laufzeit): 5.15.2 (Plattform: xcb)
  Qt-Version (bei Erstellung): 5.15.2
  OS-Version (bei Erstellung): Debian GNU/Linux 11 (bullseye)
  Python-Aufruf: python3 -tt

  under Debian

  Wolfgang