There is strange bug at the paper swatch.
The tint of the paper swatch should not be customizable, but it can happen.

Try this in the latest Indesign:
- Make a shape.
- Give it a swatch, a cmyk one which you can delete.
- Set the tint on 25%
- Than delete the swatch from the list - and let the paper swatch replace it when you delete it.
- The paper swatch takes over the 25% (though it isn't changeable after it)

When you change this CMYK-swatch to the paper swatch instead of delete it, the tint jumps properly to 100%.
But when you delete the CMYK-swatch and replace it, it takes over the 25%.

Problem is, that it occurs faults at printing. But you do not see anything on the color - though it is on 25%.