The skin of bamboo contains silica and is too smooth for epoxy to adhere.
It needs to be sanded down all the way.

I sanded the ends of my tubes before I tacked my frames together but I did not where the braze-ons mounted. I had trouble with them popping loose on my first frame.

I sanded well below the outer layer. That is why the bamboo is golden colored. When I first heat treated it, it was dark brown

Gotcha.
Maybe try misting or wiping the sanded ends down with a damp towel to open the pores of the bamboo to facilitate the epoxy penetration.

That or cut light threads in it with your dremmel.

That's basically what I ended up doing when I fixed the downtube joint.

The very outermost layer on the bamboo is sorta waxy, get rid of that and the epoxy adheres fine. Silica is the favorite thixatropic element for a lot of admixtures to epoxy.