@runaway_pancake As suggested, I've now tried this with 0.3 - 1a of resistive load. As @Grumpy_Mike and @SteveThackery already mentioned, it didn't perform very well. In all instances, dropping the input current to somewhere between 4 and 5v induced a lockup state whereby it would consume upwards of 3a until the voltage exceeded 5v again.
So, instead of a problem where I can't use it because if the input goes below 3.5v the voltage would destroy whatever is attached to it, I have a new problem whereby if the voltage drops below 5v the buck boost will potentially destroy itself/ and the battery instead.
Does anyone have any other ideas? Should I be looking for a better Buck/Boost or is this entirely the wrong approach for maintaining a stable 5v from 6v battery?