M365 account automatically drops from Edge sync on iPhone (works on iPad)

I'm experiencing a critical sync issue with Microsoft Edge on iOS where my primary M365 account will not remain signed in for sync.
The Problem:

When I attempt to sync settings, my most important M365 account does not appear in the list of available accounts
I can manually "add another account" and successfully authenticate
The account briefly appears in the sync settings
Within a few minutes, it silently disappears and Edge reverts to a default profile
No error messages are shown - it just vanishes

What Makes This Unusual:

The SAME account works perfectly in Edge on my iPad (same iOS version 18.0.1)
The account works fine in other Microsoft apps on the iPhone (Outlook, OneDrive)
I am the M365 administrator for this account - there are no restrictions, conditional access policies, or authentication blocks
The account does NOT appear in Azure AD sign-in logs when this happens, suggesting it's using iOS authentication broker

Troubleshooting Already Attempted:

Completely uninstalled and reinstalled Edge multiple times
Cleared all browsing data, cookies, and cache
Signed out of Apple ID and back in (to reset iOS keychain)
Removed iPhone device registration from Azure AD and re-registered
Re-authenticated through OneDrive using Passkey (temporarily worked, then reverted)
Cleared Safari website data for all Microsoft domains
Reset network settings
Signed out of all Microsoft apps and signed back in
Restarted iPhone multiple times

Device Information:

iPhone model: iPhone 15 Pro Max
iOS version: 26.0.1
Edge version: 142.0.3595.53
iPad (where it WORKS): iOS 26.0.1, same Edge version

Expected Behavior:
The M365 account should remain signed in for sync, just as it does on my iPad.
Actual Behavior:
Account drops after a few minutes with no error message, reverting to default profile.
This appears to be a device-specific authentication broker issue between Edge and iOS on this particular iPhone. Please investigate why Edge cannot maintain the authentication session on this device when the same account works on my iPad.