I've updated to Goodtask v6.8.2 this morning and love the look of the new macOS icons as they match the new iOS icons.
I have Goodtask permanently sitting in my dock as a replacement to Reminders.
Unfortunately, when I close down Goodtask, the default icon comes back. When I open Goodtask again, the selected alternative icon re-appears.
Is this intended or just an issue restricted by macOS?