brave-browser: Can't login to SharePoint account on Brave

Description

Brave goes into a long redirect loop when trying to login to https://unomail.sharepoint.com/. This started happening on ~Feb 28 2024, and presumably because of a website auth flow change. It does not look related to Shields or privacy protections, the breakage happens even with the following:

1. Shields down for both unomail.sharepoint.com and login.microsoftonline.com
2. Tracker, fingerprinting, HTTPS upgrades, De-AMP, debouncing all set to Disabled
3. 3P cookies globally allowed

https://unomail.sharepoint.com/ works fine on Safari, FF, Chrome, DDG. Brave only has troubles with SharePoint, regular login via login.microsoftonline.com directly works fine.

Community reports:

1. https://community.brave.com/t/unable-to-login-into-microsoft-account-in-brave-browser-but-working-fine-for-other-browsers/534041/4
2. https://community.brave.com/t/cannot-log-in-to-corporate-microsoft-sharepoint/534039

Steps to Reproduce

1. Go to https://unomail.sharepoint.com/
2. Try logging in with regular MS account

Actual result:

There’s a number of redirects between unomail.sharepoint.com and login.microsoftonline.com

The network activity looks like this:

1. POST to https://unomail.sharepoint.com/_forms/default.aspx returns a 302 to https://login.microsoftonline.com:443/f1f4be86-d048-47e8-aa26-15b01dcdb13d/oauth2/authorize?client%5Fid=...
2. GET to that redirected URL is 200
3. But then another POST request to https://unomail.sharepoint.com/_forms/default.aspx is started which gets a 302, etc.

Expected result:

Either logged in or if there’s no SharePoint account, a page that says that your account can't be found in the unomail.sharepoint.com directory.

Reproduces how often:

Always

Brave version (brave://version info)

1.65.6 Chromium: 122.0.6261.43 (Official Build) nightly (arm64) Happens on all Brave versions