With both the Illuminating Fire and Fulmination nodes specced, Surge no longer has a guaranteed critical hit. I doubt this is intended behaviour, but it seems that the fire aura generation from Fulmination is stopping the critical hit from happening.
Observations from testing:
With both nodes specced, Surge no longer has the guaranteed critical hit, but other melee attacks do.
By unspeccing just Fulmination, Surge returned to the expected behaviour of having a guaranteed critical hit.