Google Drive is asking for credentials to Google accounts that don't exist. Is this new?

My business doesn't use Google Drive, but we engage with other businesses that do.

In the past month, I've suddenly received a number of reports of documents being shared to us from Google Drive and the link demanding Google credentials to their work email account. Those Google accounts don't exist. (These shared document emails are not fraudulent.)

Previously, my users could have a code emailed to their work account, and this code was enough to prove they control the email address that was being shared to. Google would let them see the document.

When one of my users goes back in their email history and uses one of the old links, they can access the old document. Then, from that same browser, they can access the new links (that would otherwise demand credentials) and are whisked right through to the document. (All credit to them for finding this workaround. This is one of those things that is both clever and dumb.)

Did Google change some default setting lately to cause this miserable behavior? Do I need to train my users to how to train their contacts to share out of Google Drive so we can see it? I'd prefer to not have my users creating Google accounts with their work email addresses. (Thankfully, they don't believe me when I tell them this is an option.)