Keyboard Accessibility: returning to the base screen when you close a lightbox

I do work for the Federal government and my WBT courses must be 508-compliant. Part of that is keyboard accessibility. I've run myself ragged troubleshooting 508 functionality in Storyline 2; others who need to make compliant courses appear to share my struggle. I've worked most of the kinks out, but there is one thing I can't figure out or fix and it's pretty much a deal-breaker:

When I use keyboard accessibility (tab + spacebar mostly) to access and engage a button that opens a lightbox, and then I close the lightbox using the X button in the upper right corner, when I return to the base screen and hit "tab" again, the position indicator jumps into the menu (most frequently), or up to a Navigation tab in the interface that I created and positioned by the default Resources tab (sometimes), or some other tab-able/exposed object within the screen (rarely, which would be better, but still fail my course in the 508 review).

Why on earth is the cursor jumping out of the base screen and into the interface? How can I stop this? I will pretty much be "that lady" at work who researched, recommended, lobbied for, and got funding to implement a new development tool (Articulate 2) that now can't create acceptable, moderately sophisticated courses for our clients if I can't get a fix for this.  Help!

K