Values of external-id properties link to the external database entry from the Wikidata frontend. In some cases the external weblink is not working due to URL-encoding of special characters within the external identifier, such as %, &, =, and maybe others. An example is https://www.wikidata.org/wiki/Q325887#P3520 with the correct identifier W%D6LLEKLA01. The extra URL-encoding translates it to W%25D6LLEKLA01 which is not working.
I originally requested an update at wikidata:MediaWiki talk:Gadget-AuthorityControl.js, but I learnt that this gadget is no longer responsible for the linking.