Raw HTML code is displayed for Custom field description with html content in the Issue Screen:

Note that the HTML content is rendered properly in the Custom Field list as shown below:

And other custom fields with html description don't seem to be a problem (I tested multiline text, and select list types)

I found https://jira.atlassian.com/browse/JRA-28776, however the resolution there is not clear considering that HTML content is still supported on other custom field type description. Perhaps the resolution to that ticket is now obsolete?