My issue is identical to this one, and it did solve my issue when I changed the logo from .png to .ico in .desktop. However, a restart/log-out session would render the solution void. As you can see from my screenshot below, I have the image extension as .ico, my StartupWMClass is also correct, but on the plank, there is still a Pocket Casts icon and a active grey gear icon. The two should merge.
If I edit the image in the .desktop file again, it became normal again. But I would have to do it every single time I log-out or restarts. It would be nice to have a permanent solution. The issue has been persistent since 5.0 months ago, I have tried different Pocket Casts linux apps and they all have this issue. I have not encounter this issue in any other app as of yet (I didn't install many).
I have Pocket Casts pinned, but it's the same if I unpinned and start from the application menu.
Thank you for any help