Let’s say you do a model something like this.
You draw the ‘faces’ by setting up a plane through 3 points. In many cases the plane is not drawn correctly, but is offset from the points. Let’s assume that the plane is drawn correctly through the 3 points. When you select the plane in ortho, usually 1 of the three points is not on the plane surface which is too small. So if you draw to this point, the line goes somewhere else.
So to practically do something like this, you have to first check if the plane is through the 3 points; if It is offset, Move it there using the Transform - Move/Rotate/Scale command and while you are there, Scale it larger so that it will contain your points when you go to plane ortho. It will always need to be Scaled. Thus you can draw the triangle in plane.
Hope that this helps someone.
Best,
Tommy