Unable to log into Family Link with management account

Describe the bug
After attempting to log into Family Link with a management (not a supervised!) account, I'm sent back to the Family Link login page. When re-entering my credentials, I end up with a message saying that this account is already present on the device. I'm unable to get to the Family Link management interface.

To Reproduce
Steps to reproduce the behavior:

1. Start Family Link an tap on "Log in"
2. microG says that an app tried to log in to a Google account; confirm
3. The Goolge login page opens; Enter e-mail addres and passwort of a management (not supervised!) account and confirm 2FA
4. Select "I will be using this device" and agree to terms of service
5. After a few seconds, we are back at the Family Link start page
6. Again tap on "Log in"; confim in microG
7. After re-entering e-mail address, a validation error comes up, complaining that this account already exists on this device
8. When checking in the settings app, there's indeed a new "g" account with the entered e-mail address listed in the accounts section

Expected behavior
I would expect to be able to log into Family Link with my management account and to supervise my kids' accounts

System
Android Version: 12
Custom ROM: LineageOS 20.0 (lineage-20.0-20240629-nightly-beyond0lte-signed, to be precise)

No files, tests, or entry points are identified. Start by reproducing the login flow on Android 12 with the reported LineageOS and microG setup, then trace the Family Link authentication and account-creation path. Done means a management account reaches the Family Link management interface without being rejected as already present.