Dried up Ferrofluid?
I posted this up 6 months ago, why some were complaining about harsh Thiel tweeters that get non-linear distortions with sticky old Ferro Fluid that’s gone like tar, it has to be cleaned out, just don't add new fluid to a gummed up voice coil gap, you'll be wasting time and effort
A lot cheaper than buying new diaphragms. $10 for new fluid vs up to $200 each for new diaphragms, and they could be clones/fakes.
Still have to clean out the old fluid doing either.
Tweeter Ferro Fluid and how to change it.
https://www.parts-express.com/ferrofluid-100ul-general-purpose-for-1-diameter-vc--340-540
https://forum.audiogon.com/posts/1843203
https://www.speakerrepairshop.nl/en/instructions/replace-ferrofluid-in-tweeter/c-36
Cheers George