Auto beam generation

[Drew Bowering]

Hello,
I just used lilypond 1.1.58 to typeset a bit of music that switches
between 9/8 and 12/8 time a lot.  The automatic beam generation works
fine in the 9/8 section (ties 8ths into groups of 3), but as soon as I
switch to 12/8, it stops beaming.  I can force it to start again with a
'\property Voice.beamAutoEnd = "3/8"' right after my \time command. 
When it switches back to 9/8, the beam generation resumes on its own. 
Is this a bug or a feature??

Thanks
Drew Bowering