Root cause

Inside jira-components/jira-plugins/jira-gadgets-plugin/src/main/resources/atlassian-plugin.xml

There is a fragment:

<gadget key="text-gadget" name="Text" location="gadgets/text-gadget.xml" state="disabled">
        <param name="roles-required">use</param>
    </gadget>

Which as the only gadget has the state="disabled" attribute. This cause enabling it in JIRA does not give persistence through the restart.

Workaround

1. Unpack the atlassian-bundled-plugins.zip
2. Unpack the jira-gadgets-plugin-<your-jira-version>.jar
3. Open the atlassian-plugin.xml in a text editor and modify the following lines, then save it

   <gadget key="text-gadget" name="Text" location="gadgets/text-gadget.xml" state="disabled">
   

   become

   <gadget key="text-gadget" name="Text" location="gadgets/text-gadget.xml">
   

4. compile jira-gadgets-plugin-<your-jira-version> folder to .jar
5. zip the atlassian-bundled-plugins directory
6. restart JIRA