Latest beta version of firmware now has this fix. It’s taken a good 6 months for Ubiquiti to release a fix for PPPoe throughout issues.
NOTE - It’s beta firmware and will contain bugs and perhaps other issues so be careful in installing. Good part is that rollback is an option with console firmware unlike the network applications
https://community.ui.com/releases/UniFi-OS-Dream-Machines-1-11-0-16/0fb7556f-e5b7-45a1-a57b-495a4e579749#comment/7eacc5b2-1a24-4021-a9b5-2a5250a055f3