gosh! fascinating-- hope someone can explain it!

Two thoughts Are the lower leaves yellow - have you been over watering it? Or is it infested with thrips/midge? But if the other buds are ok, then perhaps its just a quirk?

This is called proliferation - the cause can be damage to the developing bud by physical injury, such as frost, or viral. As long as the whole bush doesn't have flowers like this, its hopefully not a virus.

If its viral Bamboo is it hopeless?

Not necessarily - the commonest cause is damage of some kind, but if you have a bush where all the flowers are like this 2 years running, they'll probably always be like it, and it will be a virus that's caused it. Sometimes, though, the plant gets over it and things revert to normal after a year.

Thanks B

Thanks for everyone's response, the rest of the bush is flowering normally and there are no obvious signs of frost damage, yellow leaves, virus or bugs so I am assuming it's just a quirk but if more appear I will let you know