We've had issues in older JIRA versions where if the issue summary contains a newline character, the HTML emails can get corrupted. e.g. https://jira.atlassian.com/browse/JRA-27343
Adding newlines to issue summaries isn't possible through the UI, but if issues are created from plugins or via the API, it's possible to get bad data in.
Perhaps try deleting and completely retyping the issue summary to get rid of any invisible newline characters that could have slipped in?
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