Merge Module Directory errors in 6.0.0-rc.2

[chrpai]

I've always authored merge modules like:

<?xml version="1.0" encoding="utf-8"?>
<Wix xmlns="http://wixtoolset.org/schemas/v4/wxs">
  <?define SourceDir="..\Deploy"?>
  <Module Id="MergeModule2" Language="1033" Version="1.0.0.0" Guid="89017259-7d32-44bf-9407-9843d0890751" InstallerVersion="200">
    <SummaryInformation Manufacturer="test" />
    <Directory Id="TARGETDIR" Name="SourceDir">
      <Directory Id="MergeRedirectFolder">
]      </Directory>
    </Directory>
  </Module>
</Wix>

This works fine in 5.0.2 but in 6.0.0.rc.2 I get a build error:

The Directory symbol 'TARGETDIR' conflicts with a virtual symbol. Use the 'override' access modifier to override the virtual symbol or use a different Id to avoid the conflict. Referenced from wixstd.wixlib
Location of symbol related to previous error.

Is this expected and by design or a 6.0 regression?