Or is this a software bug?

Unlikely if it is on one board only.

3. As soon as the Pyra enters a bit of powersaving mode (switching off
the backlight of the LCD), it freezes. Not sure if it only blanks the
display or does something else, but it looks like something freezes
when entering a low-power state.

Well, backlight off is just some screen saver inside X11 - no special power