try this...

Start a new blank drawing.

Draw a line on the screen that extends from one end to another at some distance but with 0 rotation so it's just a horizontal line on the screen.

Now start the Line command again and this time begin in the midpoint of the line using osnap midpoint and then with orthomode turned on click the endpoint of the line with osnap endpoint.

Do you have a perfectly horizontal line one on top of another?