There is possibly an issue with the dictionary based break iterators, in dispatching incoming characters to the correct break engine. I noticed this while looking at the code.
Each break engine maintains a UnicodeSet of characters that it handles. Dispatch of an incoming character to a break engine is done based on this set for break engines that have already been instantiated, or by the character's Script if an engine has not been instantiated. The Sets and the Script are not identical (although close), leaving a possible glitch.
Needs to be confirmed with a test.