Updating links to other spreadsheet files only appears to work when first opening a spreadsheet or when first creating or modifying a link. If the spreadsheet to which you link is opened and then data in that changed it is impossible to refresh the link without completely closing down LibreOffice.
Could this be related to https://bugs.freedesktop.org/show_bug.cgi?id=34637 ?
Ah - I hadn't spotted this bug although I had searched for it. It looks probable that this is related so I'll have a look at 3.4 to see if it resolves this issue.
*** This bug has been marked as a duplicate of bug 34637 ***