As a part of today's unattended-upgrades run, the wmf-certificates package was automatically upgraded causing the /etc/ssl/certs/Puppet_Internal_CA.pem file to be overridden with production values.
Puppet itself uses /var/lib/puppet/ssl/certs/ca.pem (untouched), but this has broken several other things, at least T296125: Fatal error: Uncaught ConfigException: Failed to load configuration from etcd