DOTX templates do not have preview thumbnail and that is this bug. DOTX templates are mentioned in bug 115190 but focus there is on the name. Still it has attachment 139321 [details] Screenshot of the template manager with an ott and a dotx template.
Microsoft formats may have a docProps/thumbnail.* image whose purpose is to be used as the thumbnail in file managers, etc. ODF files has the same idea - with a "Thumbnails\thumbnail.png" image. The templates browser currently only supports ODF files (with a Manifest.xml that indicates the presence of one of these thumbnails). A proposed patch that supports generic thumbnails found in docProps is at https://gerrit.libreoffice.org/c/core/+/184722
Justin Luth committed a patch related to this issue. It has been pushed to "master": https://git.libreoffice.org/core/commit/9ba4c479d8f510cadf1c76bab5ced4b29745bc6b tdf#166035 thumbnailview: recognize some MS-format thumbnails It will be available in 25.8.0. 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: https://wiki.documentfoundation.org/Testing_Daily_Builds Affected users are encouraged to test the fix and report feedback.
further improved by mk with https://gerrit.libreoffice.org/c/core/+/184750.