SMW Slideshow with Image File: pages using just a Category produces SMW::on and SMW::off tags.
From semantic-mediawiki.org
Fragment of a discussion from Help talk:Slideshow format
Some info about SMW::on/off can be found at [0].
I'm guessing since MW 1.25 changed the API interface (and slideshow uses the MW API) that something is missing before/after the parsing and I'm guessing further that if you would use MW 1.24 then you would not see SMW::on/off.
[0] https://github.com/SemanticMediaWiki/SemanticMediaWiki/issues/1055#issuecomment-110536701