for further clarification:
this is what Photo Mechanic's app icons look by default:
please make it look something like this:
probably you could do the new icons more fancy if you want. I just took the old ones and did a quick and dirty square version of it.
So it's not the content, just the shape. Our application supports macOS versions much older than the one you're running. If we were to put in the icon that appeals to the latest design, it would look out of place on the older macOS versions. Then we'd have those folks complaining.
Unfortunately, there is no way to have several icons, one for each macOS revision (that I know of). That said, in the future, we will not be supporting older macOS editions as much as we do now and this won't be a problem.
-Kirk