This manifestation happened after I had loaded an existing drawing, made modifications, then used print preview and adjusted the scale to fit the paper.

The file was closed. Upon reopening lines designated as .25mm were unusually thick such that two lines 2mm apart appeared as one solid line. Placing the cursor over the line showed it was comprised of two lines.

Changing the properties of a line to make it thinner did not help with one exception and that was to set the line thickness to default, when the two lines became visible.