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:
I’m a designer on the Jira team. For a long time, I’ve fielded questions from other designers about how they should be using Jira Software with their design team. I’ve also heard feedback from other ...
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