File Drop prompting for login?

Just installed NextCloud 15.05. I set up a new folder in the admin account to test a “file drop” scenario for letting people upload files for events without needing a login. When I test this out in a new browser or private window, I see the prompt to upload files, but as soon as I try to upload any files, I’m prompted to log in. That seems to defeat the purpose.

I’ve checked the global admin setting and anonymous uploads are allowed. The folder share is set to a link allowing public uploads as far as I can tell.

Any ideas what might be prompting a login? Any log settings I should enable or files to look for to help me see where this might be having issues?