It turns out that my problem comes from the Neutron defaults in Stein Packstack, which changed significantly from earlier releases. Notably, Stein Packstack defaults include ML2 mechanism driver OVN instead of Openvswitch, and type driver Geneve instead of VXLAN.

I solved my problem with an even longer packstack invocation, explicitly setting type and mechanism drivers and the default tenant network type, as well as the L2 agent:

sudo packstack --debug --allinone --default-password pw        \
               --os-neutron-ovs-bridge-interfaces=br-ex:eth0   \
               --os-neutron-ml2-tenant-network-types=vxlan     \
               --os-neutron-ml2-mechanism-drivers=openvswitch  \
               --os-neutron-ml2-type-drivers=vxlan,flat        \
               --os-neutron-l2-agent=openvswitch               \
               --provision-demo-floatrange=10.1.1.0/24         \
               --provision-demo-allocation-pools '["start=10.1.1.10,end=10.1.1.50"]' \
               --os-heat-install=y --os-heat-cfn-install=y

I suppose I could have reached the same result by replacing the five os-neutron options with a single --os-neutron-ovn-bridge-interfaces=br-ex:eth0 (note: ovn instead of ovs), but I have not tried this yet.