Check what is in the desktop file first. If the icon name is misspelled there, it won’t work with any icon pack ever.
I know how this works because the same happened to me with Thunderbird, at some release they changed the icon name in the desktop file, but it wasn’t updated in my icon packs, I expect something similar here as well. Without checking this you can’t be sure who’s fault is this, but I guess the app’s developers or maintainers messed up their desktop file some way.