MIMF

I was watching a YT vid on acoustic neck making and noticed one maker used carbon fibre along with the old square truss rod.,but he only used the carbon fibre on one side.The bass side I believe, Does anyone know the reason for this ?

I think it is best to build a balanced construction unless one has a really good reason to do otherwise. Using two different reinforcing materials in different shapes presents a lot of variables.

Typically the bass side of the neck is under less string tension than the treble side so this might seem counterintuitive. That said it would make the truss rod more active on the treble side which may be the point of the exercise. One could "lock in" more relief on the bass side but really it's silly in my opinion.

i thought it strange as well

To save time/money? There is always that little voice in our heads looking for shortcuts. I admit I have given in, usually to disastrous results.

yes that is so true. The fellow who was making this Spanish style guitar was all jigged up, making me think he was doing it as his normal practice not a one off. I just wonder what his thinking was im sure he had a great explanation oh well