I overwatered a small B. tuldoides. Leaves yellowed.

The seller said stop watering so much.

I stopped watering. I removed the yellow leaves. No more yellow leaves. I waited until the green leaves curled signalling too dry, and started watering again, but much less. No problems since.

Could be nothing wrong with way you are watering it. In the spring/early summer, many of the leaves that are dropping are just the leaves from winter that the bamboo is replacing. Is your bamboo sending out new leaves? My bamboos are currently dropping many of their leaves and sending out happy and vivid green ones.