Login from a weblink doesn't open to site

We have several web links on our company's SharePoint communications site directed to various vendors' secure login sites. All are working fine except one. This one weblink opens the vendor's secure login site, but when we enter our vendor credentials, we end up on a SharePoint site. The strange thing is when we hyperlink to the same vendor's login page versus using a web link, it works. It is just this one vendor using the web link. Any suggestions as to why this is happening with this one vendor's login site from a web link?