If you put a MapPolygon on the map and then toggle its border.width property it will appear moved until the map is zoomed in/out, or panned, or rotated, (basically until it is rerendered).
Toggling the border width is common say if you are drawing a MapPolygon and highlighting it using its border, e.g. for selection.
(The example is a duplicate I used to show the border not painting initially, but this is a different bug.)
Use this QML in the minimal_map example project.
The bug can be triggered by starting the application, and then clicking the button twice. The first time the border on the MapPolygon will thicken, then the second time it will shrink and you will be able to see the two shapes do not align any more. If you move the map in any way to cause a rerender the MapPolygon jumps back to its normal/correct place.
I tried this with the osm and itemsoverly plugin.