I posted the same issue to the mailing list, and was informed by the author that in the new IMMS, you turn on XMMS 'shuffle' mode, which IMMS detects and supersedes.
Hence, this is actually a documentation bug -- albeit an important one, because the package is useless unless the user knows this. I suggest a standard upgrade notification of changed behaviour (via debconf or whatever is the proper way to do this these days).
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