I opened a support ticket with Microsoft. At first they were unable to confirm whether the "Guest Access Expiry" policy affects external emails on "Specific People" sharing links if those external emails are not explicitly added as guest users on the tenant. However, they advised me to turn off the "Guest Access Expiry" policy and presented that as a solution to issue.
They also advised me to use the "Manage guest expiry" features to extend the access of external emails to my sharing links. However, not all emails that I share with show up in that feature, and I have seen specific emails disappear from sharing link when those emails were not present in the "Manage guest expiry" settings.
The support reps remoted onto my computer a handful of times to grab some diagnostic info on my sharing links. However, I did not receive any conclusive findings from their testing. They just continued to refer me to the "manage guest expiry" feature. I asked them a couple more times if they could confirm that the "Guest Access Expiry" policy applies to non-guest external emails on sharing links, but they still would not confirm that point.
Since the back-and-forth on the support ticket was going nowhere, I decided to set up a fresh test on my dev account. The results of previous tests I had set up were hard to evaluate, because during one test I had toggled the "Guest Access Expiry" policy after setting up the sharing link, and in a subsequent test I had shared with some of the same emails that I'd previously shared with. I saw emails disappear from both of those sharing links, but I can't be sure that wasn't related to the "Guest Access Expiry" policy. So I set up a new test link with emails that I have never shared with (and that have not been added as guest users - which I have never done in my testing). My "Guest Access Expiry" policy has been turned off for quite some time now and I will leave it off for the duration of my new test. I will continue to monitor this test link over the next few weeks to see if any emails get dropped. If so, I can be pretty confident that there is a bug involved that is unrelated to the "Guest Access Expiry" policy. If no emails disappear from the link, then that may be partial evidence that the "Guest Access Expiry" policy is in fact related.
Since the support ticket was going nowhere and my own test will take a few weeks to monitor, I asked that they archive the support ticket. In my review of the ticket I restated that I had not received confirmation yet about whether or not the "Guest Access Expiry" policy relates to non-guest external emails on sharing links. The support rep sent back this response:
As checked further, the "guest access expiry" policy in the SharePoint Admin Center only applies to guests who use sharing links or guests who have direct permissions to a SharePoint site after the guest policy is enabled. It does not apply to external emails that are referenced on OneDrive sharing links if those emails are not explicitly added as guest users.
So this finally appeared to be direct confirmation that the "Guest Access Expiry" policy should not apply to the external emails I've been sharing with (because they were never added as guests). But now I'm confused as to why they've been advising me to turn that policy off and use the "manage guest access" features to mitigate my issue.
Obviously I still have a lot of questions, but I'll see what results my new test yields and will update this post when I have more info.
