How do I stop lines snapping from 10° to 0° while all Guides are already off?

I wonder if the snapping to "10 degrees" but different length lines get different slopes is because

1) It is snapping to a horizontal or vertical guide line that coincidentally makes it snap to about 10 degrees and

2) The angle displayed next to the line is rounded. One line might be 9.5 degrees and the other 10.4 degrees, both of which will read as 10 degrees. This can be verified by looking at the rotation angle in the sidebar.

You also have to consider that this is not a CAD or drawing application. The tools are rudimentary, as are the results.

I think holding the Command key disables snaps, but I can't find the documentation for that right now.

Holding down Command does turn off snaps, while it is held down.

I have not seen anything snap by 15 degree increments. What I see is it snapping from a few degrees to 0 or from close to 45 degrees to exactly 45 degrees.

I have not seen the degrees be different. It is possible, I suppose, if their slope algorithm has a bug but it seems like such a simple thing it is hard to believe they got that wrong. Can you post a screenshot or set of screenshots that show this definitively (i.e., no eyeballing necessary)?

In the sidebar on the right (or in the Arrange inspector), you can type in the exact degrees you want. It rotates the line around its midpoint so do the rotation before positioning the line.