This is against:
https://git.opendaylight.org/gerrit/#/c/3314/10 (controller side)

https://git.opendaylight.org/gerrit/#/c/3451/2 (openflowplugin side)

Build the controller
Build openflowplugin using -nsu

Run distribution/base/target/../run.sh -debug

From POSTMAN:

PUT: http://192.168.195.157:8080/restconf/config/opendaylight-inventory:nodes/node/foo:node:1/group/125

Content-Type: application/xml

Accept: application/xml

<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<group
xmlns="urn:opendaylight:flow:inventory">
<group-type>group-select</group-type>
<group-id>125</group-id>
<buckets>
<bucket>
<watch_port>1234</watch_port>
<watch_group>123</watch_group>
<action>
<pop-vlan-action/>
</action>
<bucket-id>12</bucket-id>
<weight>123</weight>
</bucket>
</buckets>
<install>false</install>
<group-name>Foo</group-name>
<barrier>false</barrier>
</group>

What arrives at the RPC is :

2013-12-09 05:02:22.976 PST [pool-20-thread-6] INFO o.o.o.t.OpenflowpluginGroupTestServiceProvider - addGroup - AddGroupInput [_barrier=null, _buckets=null, _containerName=null, _groupId=null, _groupName=null, _groupType=null, _install=null, _node=NodeRef [_value=InstanceIdentifier [path=[org.opendaylight.yang.gen.v1.urn.opendaylight.inventory.rev130819.Nodes, org.opendaylight.yang.gen.v1.urn.opendaylight.inventory.rev130819.nodes.Node[key=NodeKey [_id=Uri [_value=foo:node:1]]]]]], _transactionId=null]

Basically, all Group values nulled out.

Additionally, while debugging its clear that the data that comes into the Transaction has all fields nulled as well. So the issue is prior to the data commit.