I know such a feeling, taking a customer request and identify their problem with a suggested solution. That is win-win situation. I always wander why such a big company could not figure out the root cause. That is why a consultant is there with a specific expertise. Good job! I hope you are going to have them as a repeat customer.
I wouldn't solely blame the designers of the product here. The toothbrush manufacturer could have had an issue with the transducer component supplier/manufacturer - i.e. the transducers themselves, being supplied to the toothbrush manufacturer were not being manufactured to the appropriate specifications/tolerances. This happens all the time when developing complex products and it is often impossible to predict lot-to-lot variances on components.
This sounds like a product that went into production way, way too fast. Just because one prototype worked well, doesn't mean it's ready for production. An engineer who thinks even slightly outside the box should have realized the transducer sub-system is a high-Q thing!