I have a scene with motion blur enabled, and some particles emitted from a volume. The particles only move very slowly; they're basically static (no new particles over time). I've baked everything, and when I render all the frames look fine except the first one, which has long motion-blur trails. I use motion blur with 50% shutter and "centered on frame". If I switch to "starts on frame" everything is fine. Is this a bug, or am I doing things wrong? Blender 2.91 nightly, but also happens on 2.83.
Test blendfile here. Load it, render frame 1, you'll see long motion blur trails. Render frame 2, it's OK. Then go back to frame 1, set Scene > Motion Blur > Position to Starts On Frame, and re-render frame 1: all OK!