Provided that Done is the final status in that workflow, I'd say the most likely cause is that said status has the property jira.issue.editable=false in order to prevent already closed issues from being edited, which also affects transitioning capability.
Should you want to get rid off of that property, you may:
Hey Community mates! Claire here from the Software Product Marketing team. We all know software development changes rapidly, and it's often tough to keep up. But from our research, we've found the h...
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