What could be causing this Chrome security warning with a custom/external protocol?

Question

I'm trying to diagnose a co-worker's issue where they are getting a security warning in Chrome when they try to open a URL with a particular custom protocol. They are seeing this warning:

I initially thought maybe it was because of the double forward slash being interpreted as HTTPs / telling the browser to use the same protocol, but apparently when that is removed it gets added back in when they submit the request.

What could be causing this warning? Could it be an issue with how we've registered the custom protocol?

Answer 1

This has been caused by a change in Chrome 87, but it will be reverted:

https://bugs.chromium.org/p/chromium/issues/detail?id=1158169#c147