Hi Marco thanks for posting! Are you using Ctrl-V or Ctrl-Shift-V to paste? (Cmd-V / Cmd-Shift-V on Mac). If you use Ctrl-Shift-V to past text into a text box which already has the correct Punjabi font selected it should remove any formatting from the source and paste in the correct font. If this doesn't work could you try turning off WebGL? This can be done by going to View > Use WebGL and un-selecting this option.
If neither of these steps resolve the issue it could be something specific to your custom font or document. In that case, please add a screenshot of the issue and the font file you uploaded. Thank you!