Thanks. The difference seems to be in the bidi version. I have been using version 13.5 dated 2013/05/28, from TeX Live 2013 pretest, while you (Wilfred) are using version 12.2 dated 2013/04/04 -- presumably from the frozen TeX Live 2012. I can actually reproduce the behaviour you're observing with bidi version 12.3 dated 2013/04/08.
Arthur
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