Basalt has a higher elongation and not as stiff as CF, so closer to S-glass in its properties. All this is better impact resistance.
Basalt is also about half the cost of CF so would have been better from a commercial perspective.
It would have suited my application a better, but I can probably make CF work too.

There seems to be a pattern forming here!

I've been speaking to some composite specialists at a university down here, and it was suggested that the issue might be related to fibre sizing (https://www.michelman.com/Industrial-Manufacturing/Reinforced-Plastic-Composites/Fiber-Sizing/), and incompatibility with certain types of resin. Basalt is used a bit in surfboard manufacturing down here, and that mostly uses polyester and vinylester resins with hand lamination.

Interestingly my really bad results have been consistently from the 330gsm fabric, whereas the 200gsm was almost passable and felt quite strong, EDIT: but I tested for failure by bending it... low and behold, there was noticeable separation between the epoxy and the fabric. I'd say it was the same as what you were seeing. This didn't happen with my CF test plate at all.
 
I'll probably do these parts in CF as I need to progress my project.