The fracture goes directly from one notch (at the bottom of the plunge) to another (file work at spine), I would look at the bottom of these features to see if there was a stress riser. But first, what steel is it? Indeed, what does the grain look like? How hot did you heat it, and with what equipment? How long did it hang in the quench tank?
I ask all of this because, although it has a reputation for being fast, I have yet to ever crack a blade in #50 that was a shallow hardening alloy for which is was designed. I know this will not be of any consolation, but if the only change you made was the quenchant, you can rest assured that you have reached a new level of hardness not previously obtained, you just need to deal with whatever uneven strain issue that is causing problems. Seriously, I have taught classes where I allowed the students to make ABS test blades with things like black automotive products or peanut oil, but then had them full quench in Parks #50, the latter blades would break and not just easily bend, which allowed them to clearly see the difference between full hardness and mix structures from inadequate quenches. Obtain maximum hardness and then the maker, not the quenchant, is in full control of the final outcome. The first variable I would address is the time hanging in the oil, a sure way to eliminate this problem is to utilize an interrupted quench method, but even without that, quenched blades should be in the temper ASAP after they assume near room temperatures if they obtained maximum hardness.
edited to add: Sorry posted at the same time as you so many questions already answered.