Overcloud deployments did not consume nor affect the Neutron mechanism driver parameter passed as hiera data from the corresponding Heat template. This meant Overcloud deployments contained an unexpected Neutron configuration, with both openvswitch and linuxbridge configured as mechanism_drivers in /etc/neutron/plugins/ml2/ml2_conf.ini as such:
mechanism_drivers = openvswitch,linuxbridge
This fix ensures the Overcloud deployment correctly consumes the neutron_mechanism_drivers hiera data item passed from the Heat templates and sets this in the Meutron ml2 configuration on the controller node. You can also specify the NeutronMechanismDrivers Heat template parameter and as a custom parameter and expect the corresponding ml2 configuration for Neutron.