1. Insert > Fields > More Fields (Ctrl+F2), choose "Document" tab.
2. In "Format" column, "Style" should be "Template name"
Reason: if the document is opened from a template, then this field displays the template name.
Seth Chaiklin committed a patch related to this issue.
It has been pushed to "master":
tdf#139295 improve label in Insert Fields dialog for type Templates
It will be available in 7.2.0.
The patch should be included in the daily builds available at
https://dev-builds.libreoffice.org/daily/ in the next 24-48 hours. More
information about daily builds can be found at:
Affected users are encouraged to test the fix and report feedback.
You likely have checked everything. Thanks for the patch.