I confirm that leaving the application open in iOS does result in the badge properly updating.
The one follow up I have is why does Good Task have to remain open for the badge to properly update when other apps such as Apple Mail, Messages, What'sApp, etc. do not need to be left open. Would be great were there a way for Good Task's badge to update similar to the above listed apps.
The reason why I am interested is that most of the time I use Good Task on my MacBook but when travelling / out I generally only have my iPhone / Apple Watch with me and it would be great were Good Task not need to be opened as I generally do not have it open.
Comments / thoughts would be greatly appreciated.