Gridlines/elements unexpected change to print darker?

I use OmniGraffle (6.6.2) to print sheets for my daily pen/paper notebook, and usually print off a bunch at a time, usually a couple week’s worth.

For 4 months they’ve printed just like I expect, and how it’s reflected in the UI. When I printed two weeks ago, it printed just like I expected. Today, the gridlines are much darker, and the “rev” element is also printing darker. It’s as if the transparency went to 100%. I didn’t change any transparency settings, and when I look at the settings, they show unchanged. The printer is the same printer I always use.

Have you experienced this kind of behavior?