"Everyone except external users" added to site Members group when Administrators are changed

[Paul Hermany]

My organization uses an OOTB SharePoint Team Site (not a group site) for sensitive HR content. We've removed all access except for a handful of users in the Members group.

Whenever we add a user as a Site Collection Administrator through the admin portal, "Everyone except external users" gets added to the Members group automatically, effectively giving the entire organization access to sensitive content.

Has anyone experienced this?

[#2]

@Paul Hermany - I am having the same experience in our test environment, looks like a bug

It happens when you add or remove users from site collection administrators from the admin portal.

[#3]

I've had the same issue reported to me by a client this morning.