Phone as a security key (#enable-web-authentication-cable-v2-support) does not work properly

[zinkkrysty]

Description

I cannot use my phone as a security key even if I enable the flag #enable-web-authentication-cable-v2-support

Steps to Reproduce

1. Navigate to brave://flags on my desktop
2. Enable the flag #enable-web-authentication-cable-v2-support
3. Navigate to brave://flags on my android brave browser
4. Enable the flag #enable-web-authentication-cable-v2-support
5. Go to a site using webauthn (such as https://webauthn.io/) from the desktop
6. Attempt to register
7. In the dialog that appears, select Your Phone
8. Follow the instructions on my phone (go to Passwords > Use phone as a security key > Scan QR code)

Actual result:

The scan QR code procedure ends up with the message "Registration Failed"

Expected result:

The scan QR code pairs my phone with the desktop browser.

Reproduces how often:

Easily reproduced

Desktop Brave version:

1.24.86 Chromium: 90.0.4430.212 (Official Build) (x86_64)
e3cd97fc771b893b7fd1879196d1215b622c2bed-refs/branch-heads/4430@{#1429}
macOS Version 11.3.1 (Build 20E241)

Android Device details:

• Install type (ARM, x86): ARM
• Device type (Phone, Tablet, Phablet): Phone
• Android version: 10

Version/Channel Information:

• Can you reproduce this issue with the current release?
• Can you reproduce this issue with the beta channel?
• Can you reproduce this issue with the nightly channel?

Other Additional Information:

• Does the issue resolve itself when disabling Brave Shields?
• Does the issue resolve itself when disabling Brave Rewards?
• Is the issue reproducible on the latest version of Chrome? No

Miscellaneous Information:

If I repeat the same procedure with Chrome, it works.
After I paired my phone in Chrome, the device also appears in Brave Desktop when attempting to use webauthn, and the notification appears on my phone, however when I press on it, an error comes up (Registration failed)