XATTR CAS Macro is incorrect

When performing a subdocument operation using Macro-Expansion to replace `${Mutation.CAS}` with the new CAS of the document, the value written into the document doesn't match the CAS of the document.

I perform a document upsert:
SET `testXattr` TO `{"x": "xattrs"}`

I perform a sub-document mutation:
SUBDOC_MUTATE `testXattr`
  DICT_SET(XATTR_PATH | MKDIR_P) `xatest.test` TO `"test value"`
  DICT_SET(XATTR_PATH | EXPAND_MACROS) `xatest.rev` TO `"${Mutation.CAS}"`
  DICT_SET() `x` TO `"x value"`

I perform a sub-document lookup:
SUBDOC_LOOKUP `testXattr`:
  GET(XATTR_PATH) `xatest`
  GET() `x`

And the results are the following:
CAS = 0x149b717d56330000
RESULT 0: `{"test":"test value","rev":"0xd80f0f20dfb94b3d"}`
RESULT 1: `"x value"`