Summary
Custom field description in Jira shows HTML tag <p> ... </p>
Steps to reproduce:
Create custom fields in the instance
Expected results:
Custom field description does not show any HTML tag <p> ... </p>
Actual results
Some custom fields' description show HTML tag <p> ... </p>

Notes
Not sure what causes this. The issue is very random and not easily reproduced, and does not happen with every instance.
Workaround
There is currently no workaround for this issue.
Sometimes turning ON the HTML at Administration > System > Find for "Enable HTML in field descriptions and list item values" and "Enable HTML in project description" > TURN ON, could fix the issue, as the tag is interpreted rather than displayed as text, however, some Customers cannot enable this due to security reasons.