I understand that you are trying to customize the introduction gadget that exists on a dashboard in Jira Server. However this specific location is not intended to use wiki mark-up for formatting that text.
When you edit this in the System -> General Configuration, that introduction section mentions this uses HTML instead of wiki markup:
A short introduction message displayed on the dashboard.
You can include HTML, make sure to close all your tags.
This is also explained in the Configuration Jira application options documentation.
Your screenshot looks different than mine. In my configuration, I don't see the option to preview my text on the system setting page. Instead I can only enter it there, and update. Then I have to return to the dashboard to see my formatted content.
I am not sure why your site is showing that preview option. My Jira 8.1.0 test site doesn't do that here. Perhaps it's your version of Jira or the combination of the language pack you might currently have set for Jira. I am interested to learn more about your environment so that I can better understand this problem.
I also hope this helps to clarify the expected functionality here.
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