When we try to create and merge with an absolute target path, it works well.
The namespace can be solved based on the value of the operation.
Request as below:
Request as below:
PATCH http://localhost:8181/restconf/config/network-topology:network-topology/topology/topology-netconf/node/iosxrv-7/yang-ext:mount/
Payload as below:
<yang-patch xmlns="urn:ietf:params:xml:ns:yang:ietf-yang-patch">
<patch-id>VPN-CREATION-1</patch-id>
<comment>Create VPN-1</comment>
<edit>
<edit-id>1</edit-id>
<operation>create</operation>
<target>/Cisco-IOS-XR-infra-rsi-cfg:vrfs/vrf</target>
<value>
<vrf xmlns="http://cisco.com/ns/yang/Cisco-IOS-XR-infra-rsi-cfg">
<vrf-name>vpn-78</vrf-name>
<create></create>
<afs>
<af>
<af-name>ipv6</af-name>
<saf-name>unicast</saf-name>
<topology-name>default</topology-name>
<create></create>
</af>
<af>
<af-name>ipv4</af-name>
<saf-name>unicast</saf-name>
<topology-name>default</topology-name>
<create></create>
<bgp xmlns="http://cisco.com/ns/yang/Cisco-IOS-XR-ipv4-bgp-cfg">
<import-route-targets>
<route-targets>
<route-target>
<type>as</type>
<as-or-four-byte-as>
<as-xx>0</as-xx>
<as>4804</as>
<as-index>1</as-index>
<stitching-rt>0</stitching-rt>
</as-or-four-byte-as>
</route-target>
</route-targets>
</import-route-targets>
<export-route-targets>
<route-targets>
<route-target>
<type>as</type>
<as-or-four-byte-as>
<as-xx>0</as-xx>
<as>4804</as>
<as-index>1</as-index>
<stitching-rt>0</stitching-rt>
</as-or-four-byte-as>
</route-target>
</route-targets>
</export-route-targets>
</bgp>
</af>
</afs>
<description>vrf for VPN which connects each site from iosxrv-7 and iosxrv-8</description>
<vpn-id>
<vpn-oui>4804</vpn-oui>
<vpn-index>1</vpn-index>
</vpn-id>
</vrf>
</value>
</edit>
<edit>
<edit-id>2</edit-id>
<operation>create</operation>
<target>/Cisco-IOS-XR-ifmgr-cfg:interface-configurations/interface-configuration</target>
<value>
<interface-configuration xmlns="http://cisco.com/ns/yang/Cisco-IOS-XR-ifmgr-cfg">
<active>act</active>
<interface-name>Loopback1</interface-name>
<interface-virtual></interface-virtual>
<ipv4-network xmlns="http://cisco.com/ns/yang/Cisco-IOS-XR-ipv4-io-cfg">
<addresses>
<primary>
<address>7.7.7.7</address>
<netmask>255.255.255.255</netmask>
</primary>
</addresses>
</ipv4-network>
<vrf xmlns="http://cisco.com/ns/yang/Cisco-IOS-XR-infra-rsi-cfg">vpn-78</vrf>
</interface-configuration>
</value>
</edit>
<edit>
<edit-id>3</edit-id>
<operation>merge</operation>
<target>/Cisco-IOS-XR-ipv4-bgp-cfg:bgp/instance/default/instance-as/0/four-byte-as/4804/vrfs/vrf</target>
<value>
<vrf xmlns="http://cisco.com/ns/yang/Cisco-IOS-XR-ipv4-bgp-cfg">
<vrf-name>vpn-78</vrf-name>
<vrf-global>
<route-distinguisher>
<as-xx>0</as-xx>
<as>4804</as>
<type>as</type>
<as-index>1</as-index>
</route-distinguisher>
<exists></exists>
<vrf-global-afs>
<vrf-global-af>
<af-name>ipv4-unicast</af-name>
<enable></enable>
<connected-routes></connected-routes>
<static-routes></static-routes>
</vrf-global-af>
</vrf-global-afs>
</vrf-global>
</vrf>
</value>
</edit>
</yang-patch>

However, when we try to delete or remove with an absolute target path, restconf returns an error of 400. Message is "Error parsing input: null".
Request as below:
PATCH http://localhost:8181/restconf/config/network-topology:network-topology/topology/topology-netconf/node/iosxrv-7/yang-ext:mount/
Payload as below:
<yang-patch xmlns="urn:ietf:params:xml:ns:yang:ietf-yang-patch">
<patch-id>VPN-DELETION-1</patch-id>
<comment>Delete VPN-1</comment>
<edit>
<edit-id>1</edit-id>
<operation>remove</operation>
<target>/Cisco-IOS-XR-ipv4-bgp-cfg:bgp/instance/default/instance-as/0/four-byte-as/4804/vrfs/vrf/vpn-78</target>
</edit>
</yang-patch>

According to my inspection, it can be located to org.opendaylight.netconf.sal.rest.impl.XmlToPATCHBodyReader:147-154
The namespace is resolved to "netconf:base:1.0" because the request URI is root and there is no value can be used to resolve the namespace.