WKED_B (Mode=0Ah) Set the bit to 1 to sense failing edge, clear to 0 to sense rising edge, not vice versa as stated in the SX Datasheet page 9

The only way to get the SX to generate an interrupt when the comparator output at RB0 goes from low to high would be to connect the output pin to another pin on port B configured as an interrupt with its interrupt enabled. In other words, the interrupt for MIWU on port B does not also work when the port direction is set to output.