Do you use iCloud to manage Reminders database? - yes. I didn’t know there was another option. I assumed it uses the reminders by default.
Would you check default Reminders app to see if data is shown properly? - yes. No issues here.
Would you try changing priority on default Reminders app and see how it goes? - no issues.
If it worked fine, would you try it on GoodTask on same task? - still crashes.
Does all tasks behave like this? If not, is there anything different about tasks that's crashing? - no. Other tasks do not have this issue. The task had a location set on it, but so did other tasks I tested. What’s funny is that I removed the location within GoodTask and then the issue went away. I readied the location and the issue is no more. Weird.
Does other changes rather than priority changing work well? - yes. The other settings didn’t cause any problem.