I checked the character, it is anything but weird. It is a valid character in a language used by millions of people.
For those curious: It is a Telugu character; Telugu is one of several widely spoken languages in India.
I guess it depends on your definition of "widely spoken."
According to Wikipedia, the number of people speaking Telugu is 0.97% of the world. It's not even a statistical margin of error.
Still, how hard can it be to have a machine step through all of the possible combinations of every iOS-supported character set and jam them into iMessage to see if they're failsafe?
However the crash also occurs with a Bengali sequence, and ZWNJ does have an effect in Bengali, so that sequence is not "weird", if perhaps rare.