See a demo of the problem here. https://www.mediawiki.org/wiki/User:Halfak_(WMF)/Page_link_demo
On the sandbox wikipage, you can see that in the first image link, the first slide of the PDF is rendered as a thumbnail. In the second link, page 33 of the PDF is rendered.
But on the Flow page, if you try to paste [[File:....pdf|thumb|right|300px|page=33]], the link will be changed to [[File:....pdf|thumb|right|300px|]] on preview or save. The |page=33 is removed and the first page of the PDF is rendered.