The strike-out appears when an issue is seen as "resolved" by Jira.
That is done by looking at the Resolution field. If there is anything in the field, the issue is resolved. It is only unresolved when the field is empty (null in the database). Note that Jira displays "Unresolved" in the field display when the field is empty.
You need to clear out the value, and the only way to do that (without code) is to push it through a transition that has the standard "clear resolution field" post-function. That is usually placed on transitions that are "re-open" type things.
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