bug in repeated file loading / downmixing to stereo with 2.6.4

[jep]

First off, I'm not sure if this is the right place to post this, since it's labeled 2.5 discussion.  But I couldn't find anyplace for 2.6 discussion.  Moderators please move it if there's someplace better.

I have a stack of MKV files with a single Dolby AC3 48000Hz 6ch track (if it matters, the video is MPEG4 (H264) 1280x720 25fps).  I open one and choose AAC (Faac) and set the Filters to remix stereo.  Then I save the file and all goes correctly.

But then I open ANOTHER file and do the same thing, it does nothing (useful) to the audio.  The output says Dolby AC3 48000Hz 6ch.  It does SOMETHING to the file, as there's 6K difference between the size of the input and output.  But I can't tell what.  It' certainly isn't downmixing, though.

I've found that this is entirely reproducible.  It never breaks on the first file loaded, but it always does on subsequent ones.

This is in 2.6.4 on Windows 8 (I know).  Not sure what other information might be helpful.