Thats a problem with interactive PDF format. Different readers, plugins, extensions etc supports interactivity differently. And you as a publisher can´t influence how your users will see your PDFs. Only Adobe Reader and Acrobat have a full support for interactive features.
Flash format can handle that sort of things way better.. check for instance this example:
http://www.godcgo.com/Portals/0/GettingAroundguide/index.html
Whole content of that interactive guide has been produced with InDesign, but instead of exporting it as Interactive PDF it has been exported as SWF and finalized with product called eDocker*.
As you can see, there´s lots of differently shaped buttons and they work as they should - in all browsers.
I know that all designers don´t want to use flash because it doesn´t work in mobile devices, but IMO if target audience is using computers and you are using InDesign for ceating your interactive publication, Flash/SWF is very good way to deliver it to end-users. Small con is that InDesign´s own SWF export is not very good alone, you can´t make ready-to-use stuff with it without using 3rd party tools like eDocker or at least forking the code InDesign produces.