Sibelius 6 added 1-, 2-, and 4-bar repeat symbols that actually play back properly, or so I thought.

I have a piece with a VDL marimba, and the marimba part is basically a 4-bar phrase repeated over and over again. The entire marimba part has an 8va line spanning from the first marimba note to the last bar of the piece.

Where the 4-bar phrase is written out the first time, it plays correctly, one octave above where it is notated. Then the playback gets to the 4-bar repeat, and it plays back an additional octave higher (2 octaves above notated). In this case this takes the instrument into the cymbal sounds high above the staff, but I believe that it irrelevant as far as the problem goes.

When the playback hits written-out notes again, the playback is still an octave too high, and it stays that way for the rest of the piece. So, I've got nine of these four bar phrases, and eight of them are playing back incorrectly.

Is this a Sibelius problem, a VDL problem, or a problem with both working together in this manner?

You only need to have the 8va line over the measures that are to be repeated, not the repeat symbols themselves. With it notated the way you're describing, the first time it repeats, it's behaving it should.

So it's a Sibelius thing, but not incorrect, technically.