This is a classic alpha blending issue. You have to tell Panda which transparency mode to use. You can use setTransparency(MAlpha) for simple transparency, MBinary for perfect transparency without semitransparency, MDual for a smarter version of MAlpha, or MMultisample for alpha-to-coverage transparency.
This is equivalent to switching to the “Blender Game” mode at the top bar in Blender, going to your material settings, and changing “Alpha Blend” under “Game Settings”.