Modern burners are tuned for high speed burning. While 4x is not ‘high speed’, even for DL media, it’s much better in this context than 2.4x.
That’s why most modern burners achieve better results with MKM001 at 4x (and faster) than the rated 2.4x speed. Slower is not always better, this is just a myth.
I’d stick to 4x in future. You probably got a worse / less consistent batch of MKM001s, and your burner does not seem to be able to achieve acceptable results with them at the rated 2.4x speed.