One of the seams on my Naish 16M kite has started to come apart on the seam on the tube at the juncture of the second tube from the centre tube. On the back side (opposite the leading edge.

I tried regular clothing thread which I've used for the skin repair but it let go.

What type of thread should I use to repair this area?

V92 seems to be the standard sail makers go to thread.

I use dental floss since forever. Works perfect and only drawback is that it's white so the sewing is visible. Use the nylon type (some are plasticky film type and break easily)

never use regular sewing thread, it is probably cotton and has no UV protection, plus it is weak
use polyester thread preferably with UV protection, if they say outdoor use, most likely have some UV blockers.. For the really good stuff check sailrite.com

I just take an old kiteline and extract single strands from it and sew with that, probably stronger than anything else

V92 seems like overkill to me. Its sail repair thread and quite heavy. V46 would be easier to work with. But really any white polyester thread will do. Poly is strong, UV and mildew resistant. Don't use nylon.

v92 is for leading edge and strut repair. v46 for canopy.