EBU STL subtitles not visible on Windows

I experience a strange problem with EBU STL files with the current Git version (Linux) and the last nightly build (Win):

Linux: subtitles are loaded and rendered as expected
Windows: subtitles are loaded, but not visible; I checked Win 7 (64bit) and WinXP SP3.

However with the same setup, TTML subtitles work on both platforms as expected.

I attached an affected EBU STL file.

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