Accessibility Issues in Cp.17- Tabbing Order

The Z-order in my timeline and the Tab Order in Properties are set correctly. However, when I preview or publish the course, the tab behaves weirdly. For instance, I created a slide that has three buttons to display text when users click on them individually, and a button at the bottom to close this slide. When I preview this slide, the tab focus goes to the first button of the three, then jumps to the Close button before it goes to the other two buttons. The Timeline order is button 1, then 2, then 3 and the Close button at the end. The Tab Order in the Property Inspector has the same order. Tabbing is a major issue for 508 in my agency and I don't know what else should I do to enforce the right tabbing order.

Thanks for any tip.