On my new Mac (more on that later), I’ve noticed that some apps (in my case Slack and Mattermost) would revert to big gray question mark icons when I restarted them.
The key piece of information that set me on the right path was a comment on the Vandyke support forum.
While that solution worked for me, it seemed a bit too geeky. When you copied the app from the DMG or Download folder into Applications, Finder should have taken care of this for you.
My simpler solution: quit the application, drag it out of the Applications folder (to the desktop or another place, not important), and then copy it back to Applications.
This worked for me.