Upon committing a large batch of BGP peers into the configuration (~40), it appears that FRR configures the peers, but doesn't apply the complete configuration for each peer in order, thus resulting in filters not being correctly applied and too many routes being sent, upsetting peers!
The problem resolves itself eventually once the config is completely applied, but not before causing problems upstream.
This smells a bit like the old issues at startup where FRR starts without complete config/filters in place.