[BP 5.5.4] 0 CAS value returned in successful Append response

When sending an Append (0x0e) request to the server I'm seeing a status of success with a CAS value of 0 when the MutationSeqNo Hello Feature is set. When the MutationSeqNo Hello Feature is not set then the CAS value is correct and everything looks fine.

When the CAS value is 0 the packet looks a bit mangled:

0000 18 0e 03 00 10 00 00 00 00 00 00 13 00 00 00 0c ........ ........
0010 00 00 00 00 00 00 00 00 02 00 0c 00 00 a7 72 21 ........ ......r!
0020 02 98 47 00 00 00 00 00 00 00 05                ..G..... ...

The Wireshark output is attached.