Viewing PDF attachments on iphone

I have migrated a large number of notebooks with PDF attachments from Evernote into the desktop app and synchronised them via webdav with encryption enabled.

I have then synchronised to the ios app (with attachment download behavior = always) and can see the notebook tree and notes. However I am unable to open the PDF attachments. I see a white icon with a downwards arrow and a link to the PDF, however tapping on it seems to do nothing.

Versions: ios version 10.0.47 on iPhone.
Desktop 1.0.216 on Windows

It looks like this is related to the “attachment download behavior” setting - if I switch this to “Manual” I am able to tap on the download icon and see the attachment.