Under Manjaro Linux, in the task manager of the default panel, instead of the icons related to specific applications, an icon appears that means something like "unknown application" or "application without system icon", which is namely the cogwheel icon in the case of the Oxygen icon set and a green icon (I'm not sure how to describe this exactly) in the case of the Breeze icon set.

The affected applications include Pale Moon, VirtualBox, and Freeplane.

The icons are shown correctly everywhere else, by the way. Also, the issue persists regardless of which icon theme or desktop theme is being used and whether the icon-only or the regular task manager (with labels) is active.

Reproducible: Always

Steps to Reproduce:
1. Launch an application such as Pale Moon, VirtualBox, and Freeplane.

Actual Results:  
Instead of their actual icon, a cogwheel icon (in the case of the Oxygen theme) or something incorrect appears in the case of all these applications in the task manager.