tts:writingMode="rl" with latin characters and a comma

Hi all,

The WritingMode007.ttml test [1] specifies tts:writingMode="rl" on
latin text that contains a COMMA.

[1] https://github.com/w3c/imsc-tests/blob/master/imsc1/ttml/writingMode/WritingMode007.ttml

Is the render at [2] expected? Specifically, the latin characters are
start-aligned but ordered left-to-right, with the exception of the
COMMA, which is ordered right-to-left.

[2] https://github.com/w3c/imsc-tests/blob/master/imsc1/png/WritingMode007/0.000000.png

Thanks,

-- Pierre