https://rt-sso.linuxfoundation.org/Ticket/Display.html?id=63631
https://rt-sso.linuxfoundation.org/Ticket/Display.html?id=62591

Issue is that AS SOON AS RESOLUTION IS SET TO SOMETHING, then JIRA will consider this item as done, hence the strikethrough.

But there is a workaround, to fix it though.

I verified these steps on the old system earlier.

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Strikethrough comes as soon as the RESOLUTION field has manually been changed/updated.

To clear it, the following link was used.
https://confluence.atlassian.com/jirakb/howto-bulk-clear-resolution-635896313.html

HowTo: Bulk Clear Resolution
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Sometimes, a situation can cause Issues to be resolved (the Resolution field being set) when it wasn't actually desired. These instructions will provide a practical (if time-consuming) method to clear the Resolution field of these Issues. The steps include creating a transition and then bulk-editing your issues with this transition to clear the field value. Note that a transition will need to be added to each status in which Issues need to be updated.
1. Prepare your transition(s).
Start by Editing your existing Workflow.
In the Diagram view, click Add transition.
To status and From status should have the same value, so the transition will be a loop (Issue status won't change);
Name should indicate the action to be performed, e.g. "Clear Resolution".
Click the new loop transition to display its properties, and click Post functions.
click Add post function
select Clear Field Value and click Add
select the Resolution field and click Add
Repeat steps 2 and 3 for each status for which the Resolution field will need to be cleared.
2. Transition your Issues
As this will work for one Status at a time, completing the operation may require multiple iterations of the following steps:

Using the Issue Navigator, locate the list of Issues you'll need to clear Resolution for

Click Tools > Bulk Change (N Issues)
Select Issues to change and click Next
Select Transition Issues and click Next
Select your newly created transition and click Next
You can clear the 'Send mail for this update' checkbox to prevent email notifications from being sent
In the operation summary screen, click Confirm to apply the changes