This is a known bug introduced with firmware version 07.01.2. We're currently working on a fix to patch up the issue.
If you'd like to continue using this firmware version and make PPPoE work, you may try to set the PPPoE values using "uci" commands via CLI (SSH). Please note, however, that any change to the PPPoE interface in the WebUI will result in username/password values becoming empty again.
The relevant uci PPPoE configuration commands should be the following:
uci set network.<PPPoE_Interface_Name>.username='[email protected]'
uci set network.<PPPoE_Interface_Name>.password='yourpassword'
uci commit network && /etc/init.d/network reload
Make sure to enter the name of the interface without the <> brackets.