After some investigation, it would appear that if you have any transforms applied to a shape, such as a stretched triangle, the Rotate Copies LPE effect will itself also be tranformed. This is perhaps something which could be considered a bug. It's unexpected behaviour at the very least.
So, with that in mind, here are a couple of possible fixes, all of which are simply designed to avoid transforms altogether:
- Draw the triangle using the Stars and Polygon tool, do Path > Object to path, but then instead of stretching it, use the Edit Paths by Nodes tool to change the shape of the triangle, thus avoiding a transform.
- Draw the triangle using the Bézier tool instead to avoid creating any transforms. In other words, don't squish or stretch the shape.
- Use Extensions > Modify Path > Apply Transform on a shape you already transformed, to remove the transform before applying the effect.
Now it should work as expected.