Missing icons in dock/plank

In my dock both the Thunderbird and “Applications” icons are missing.

When I change the icon theme, the icons of Thunderbird and “Applications” are also missing in other popular themes (arc, breath, adwaita).
I use the Breeze icons theme.

However the Thunderbird icon is not missing in the custom Papirus theme (but this is not the original Thunderbird logo)

  • I tried reinstalling Thunderbird, which did not solve the issue.
  • I tried running gtk-update-icon-cache in terminal, this also did not solve the issue.

The original Thunderbird icon is visible in the start menu and in Albert menu.