choose whether and when to clean up STL geometries
STL files contain the node coordinates for each triangle, so node points that belong to multiple triangles are saved repeatedly. During cleanup, node points are clustered if they are very close to each other and afterwards, degenerate triangles are deleted. If there are other geometry items have been loaded before (e.g. via include{} or as PlainBoundaryElements), then this can lead to problems.
Thus, by default, the STL clean up is only done as long as no other geometry has been loaded. To change the behavior, adapt this parameter.