A patch to replace IPv4 nexthop on IPv6 sessions with a MPLS labelled nexthop where the IPv4 address is encoded as an IPv6 hop breaks classic multi-protocol BGP.
The upstream patch was reverted, whilst the patch is being refactored to skip this behaviour in various scenarios (including route reflectors, route servers and when the IPv6 nexthop is a global IPv6 address).
Kindly requesting that the patch to revert this behaviour get included in VyOS 1.5 nightly:
https://github.com/FRRouting/frr/commit/e575bd8e869e7cf208b45250314587d7d1e3782a
Essentially reverts FRR commit 0325116 (https://github.com/FRRouting/frr/commit/0325116a27258e1df773a046e8668a029bead60c)