I noticed you said that you set the vertical alignment to justify. That is what is causing the problem. David’s first link explains that. Basically–your are overriding the leading values. I’m not sure if this page justified because of a bad break (i.e., widow/orphan) or if the straddle head is not on even lines.
I’m not sure how to explain it, but you always want your leading to add up.
For example, let’s say your leading is 12 for body text. And let’s say you have a 2-line display head that is on 14 lead. That means the depth is now off by two points. So you have to have the space above/below the head add up to 12’s (i.e., 12, 24, 36). So if you had 10 points space above the head, you’d be back to even lines (14 + 10 = 24).
I always use absolute leading and don’t have vertical alignment justified, nor do I lock things to baseline grid. It’s all about the leading and the space above/below different elements adding up.
I’ll look around and see if there is an article on it. But hopefully someone else can chime in and explain it better than I am trying to.