This is unusual but it could be some sort of gall, which is an abnormal growth of plant tissue formed in response to a wound or insect activity. As you mentioned that the flesh was bright green when you cut into it, it is unlikely to be a fungal or bacterial infection. It could be the activity of gall mites - tiny microscopic insects which secrete chemicals that cause abnormal growth. It is common to see galls forming at this time of year due to the egg-laying activities of the mites. Although they look unsightly gall mites rarely harm the plant. If you only have a few galls you can simply remove the affected shoots - there are no chemical controls. If this isn’t feasible you can just leave the galls as they are, they won’t hurt your mock orange.
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