The reason behind this is that almost every dance mat sends also D-PAD actions to make it work in game menus. This is also the default mapping in BeatX.
As D-PAD is by definition a directional rocker that is mechanically connected to a center pivot point. Therefore there is no mapping for Up/Down - Left/Right (jumps)
Gladly the dance mats send also Joystick or Keyboard button actions which are independent so you can map them in BeatX (you just have to do it your self as every dance mat has them assigned differently). Just go to Settings/Controls and map the controls.
NOTE: Android has trouble detecting the key press of some dance mat's (So the fix won't work). Unfortunately this also includes the classic PS3 Konami dance mat. I'm still working on a at least a partial fix (as this is the most common dance mat out there). If you own this dance mat and want to help - please contact me via email@example.com