The PPPoE peer template's connect-on-demand branch tests default_route, a key removed by migration, so it is always false.
Location
data/templates/pppoe/peer.j2:75-84
src/migration-scripts/interfaces/26-to-27
python/vyos/ifconfig/pppoe.py:121
Problem
'auto' in default_route is always false under ChainableUndefined.