Currently, the NULL snat case is not supported by OVS userspace conntrack, this needs to be added, especially to support the BZ1910378 use case with DPDK. See BZ1910378, and BZ1935663 for more details.
v4 sent upstream on April 12th http://patchwork.ozlabs.org/project/openvswitch/patch/161824216710.172116.15352631674506299358.stgit@fed.void/
v6 http://patchwork.ozlabs.org/project/openvswitch/list/?series=241223&state=%2A&archive=both
v8 http://patchwork.ozlabs.org/project/openvswitch/list/?series=252124&archive=both&state=*
Patch got accepted upstream for 2.16. It will not be backported because it's considered a new feature.