Dear Xara...

...Bug - When you place a rollover button on top of a pop-up (Highslide) image, the Highslide pop-up action hijacks the expected mouseover action.

It is fine for a single Link.
If you add a Web Animation > Reveal Effect of 0 Seconds Speed, it works. This is a definite fudge and tedious to implement as the effect still fires. It does not work for v12.
If you Name the button htmlclass='hs-btn' and add CSS code to the Website of <style .hs-btn {z-index: 100; } </style>, this sets things straight.

I can only assume this Bug has been around forever.

Highslide - Button FAIL.xar