Sometime last year, an update was pushed corporately that started breaking all S123 links sent via Outlook.
The issue was that the second double slash (highlighted below) was being truncated automatically by Outlook.
https://survey123.arcgis.app?itemID=892c04cfda4144ebb1c06d6eae6af137&portalUrl=https ://fqdn.com/portalAdaptor
Resulting in this screen (notice the missing / )

The short-term fix was to tell users to manually copy-paste the URL text into a browser from Outlook.
Well, I finally found the KB article to fix that silliness. After applying the Registry fix on my PC, it appears that all S123 links I send now work on my colleagues machines (i.e., the registry fix doesn't necessarily need to be deployed corporately).
I had a case open with Esri when this initially started happening since it was a sudden change. We were unable to find a cause at the time. I'm unsure if others have run into this, but for me at least this is a big sigh of relief!