The Scancpeak 8513 (used by Proac) tweeters have a foam ring applied to the faceplate, over time this foam can deteriorate just in the way you describe, New foam pads can be bought I think from Madisound, they are self adhesive - you can clan off the old ones with Goof Off or similar. Make sure to keep this off the dome when cleaning it off. This foam is for diffraction only and really have no bearing on the "functionality' of the tweeter.
Best of luck
Peter
Best of luck
Peter