If a Be tweeter is properly constructed, there won't be any ringing in the audible band and well above it. While it has very low internal damping, implemented properly it is considered what would be called high damping, so rigid that it really doesn't resonate.
Now as for those delayed energy charts, look through all the measurements and read the comments. In the older Utopia, the mid driver wasn't well isolated and was impacting the tweeter and it does reach its breakup point is at 22khz and certainly does show. So there is a number of issues that design resulting in that measurement. Focal also didn't develop a great driver in this case as we have seen many other metal drivers of lesser material measure extremely well in the decay plots. Even Al, which should be one of the worst materials but has resulted in some of the better measurements is mainly due to the quality of implementation. The material certainly has an impact, but its use in design matters more.
https://www.stereophile.com/content/kef-r700-loudspeaker-measurementsThe Kef R700 uses a plebeian Al dome, but due to a designed shape and stiffening ring, measures perfectly clean. Kef uses just an alloyed version in the Reference and Blade series and gets the breakup point beyond 30khz. Associating a sound with material is for the most part is invalid and proper design and engineering is the correct answer. You can just as easily implement a fabric dome poorly and have messy decay plot due to energy coming elsewhere.
Those Focal speakers didn't achieve their designed goals and they likely knew it. Reached the end of the cycle and budget and released the product as it was, being the best they knew how to produce at the given time and a given cost. Looking at the Sopra 3, they still haven't quite gone as far as some other companies in spectral decay, but progress has been certainly made.
