This article documents a known issue with adding categories, tags or related assets to files in the Document and Media library. When you try to do this, a new version of the document is not created and the time stamp is not updated.
Resolution
With the introduction of LPS-37702 (document-management-10-6130), changes to categories, tags and related assets are not reflected in the document version. Therefore, if these are updated, a new version will not be created and the time stamp will not be updated.
If there is a need to make sure all changes made to the files in the Doc and Media portlet result in a new version, you can set dl.file.entry.version.policy=0.
As a reference, here is the portal.property:
# # Set this to 0 to force all updates to file entries to result in a new # version. Set this to 1 to keep the version number if no changes are # detected or if the only change is from a zero byte binary to a non-zero # byte binary. # dl.file.entry.version.policy=1