That is interesting. While I can't speak to exactly why it's doing that without digging into your specific instance, if you want to investigate you can submit a support ticket.
First thing I would try to solve it on my own would be to change the resolve project back to in progress and then complete to try and trigger a new update to see if it was just a glitch the first time around. Otherwise, you could unlink the resolvable object and then update the request on it's own.
https://experienceleague.adobe.com/docs/workfront/using/manage-work/issues/convert-issues/unlink-issues-from-resolvable-objects.html?lang=en
Unlink an issue from a project, task, or issue
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Go to the issue that is linked to a project, task, or issue.
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Click the Issue Details section.
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Go to the Overview area of the Issue Details section.
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In the Resolved By field, remove the resolvable object type.
An issue can be resolved by a project, task or issue.
This removes the resolving object from the issue.
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Click Save Changes.
The issue is no longer linked to a project, task, or issue, and you can now resolve the issue independently.
Unlinking the resolving project, updating the status and then relinking the project has done the job 🙂 (bit of a loop hole, I spoke to support and they advised it is a bug at the moment)