I've actually resolved this but figured I'd post it here in case anyone else has encountered it. I saw the issue in Chrome and not in other browsers but it might not be browser-specific, see Solution below.
I've had "Jira Labs" "New Issue View" enabled for several weeks. It can be configured here: https://<my-org>.atlassian.net/secure/ViewPersonalSettings.jspa
When clicking on an issue card in sprint view the issue modal would open and then immediately collapse down to just the modal header bar.
Solution:
After deleting local & session storage, cookies and emptying the cache the issue view modal is behaving as expected.
