Seems like potential BFD IPv6 configuration typo in vyos-1x. Diff below:
$ diff ./src/conf_mode/protocols_bfd.py ~/protocols_bfd.py
52c52
< if 'source' not in peer_config or len(peer_config['source'] < 2):
---
> if 'source' not in peer_config or len(peer_config['source']) < 2: