Strange issue with certain media gallery applications not triggering download of pdf files

We have a strange issue with a media gallery in a specific group, the users can see the download buttons but when clicked the url is swapped to include a /download suffix rather than actually downloading the uploaded pdf file

The pdf is displayed via the document viewer, but just cannot be downloaded via the button


 

so for example the url 

communities-volunteers/m/good-practice-guides/410
becomes
communities-volunteers/m/good-practice-guides/410/download

There are other groups on the same site which work fine, can anyone advise what might be wrong/misconfigured ?