I'm not sure, I didn't set up our Jira account but it looks like default behavior based on this screenshot: https://community.atlassian.com/t5/image/serverpage/image-id...

That "Issue Type" dropdown box is what I changed a value for and it cleared the description below it (also seen in the screenshot).

Our set up might be pretty basic because the A to B from my original comment was "New Feature" to "Improvement" which is included in that screenshot.