This can happen if you have Zotero installed on your browser. Zotero has an intelligent proxy detection feature and figures out that the Strauss Library has a proxied version of PubMed, even if using the public links (or raw URL).

You can turn this off by configuring your settings to "Not Autodetect" proxied hosts.

Read more about Zotero's Proxy Preferences here: https://www.zotero.org/support/connector_preferences