Normally, when authenticating to a web application, you immediately get the windows security prompt (to enter your Active Directory credentials) - the expected behavior:

However, on a subset of the web applications (on that same SharePoint server), you get a two-part credential prompt - the unexpected behavior. First a drop-down menu:

After selecting, say the first "Windows Authentication" option, you are then presented with a sign-in for your credentials (i.e., AD\username and password):

Can anyone provide suggestions on where to start the problem solving process? I.e., all the web apps should have the first "expected" behavior.
The attributes of both sites appear to be the same (both in the SharePoint and IIS consoles).
Thanks!
Note: At the request of the moderator, my original post (https://answers.microsoft.com/en-us/msoffice/forum/msoffice_sharepoint-mso_winother-mso_o365b/sharepoint-2019-on-premise-authentication/a1a33ea3-6b7a-4297-a477-002b2b772ddf) has been reposted in this forum.


