Your unit tests would have to go through 1.71650179e30 sequences to be guaranteed to catch this one. At a test rate of 1 millisecond per sequence, that's just 4×10^9 × the age of the universe, according to wolfram alpha.
Allows for 17 planes, but only small portion of those are actually used. According to Wikipedia[1], currently Unicode has 148944 codepoints + 128k private use ones (which might, or might not make sense to include in unit tests). So your time estimate is off by mere 5 orders of magnitude.
Doesn't really change the result, imho.
Are there just enough people using iOS that these sorts of bugs can be found by mistake, or is someone fuzzing CoreText? Perhaps that can be applied to provide some kind of test coverage? Even if it’s not complete?
However I'm not aware of any such things in Telugu aside from explicit virama-showing which rarely exists in input methods (and doesn't end up with zwnj in the position shown here, but that could have happened after editing).
https://en.wiktionary.org/wiki/%E0%B0%9C%E0%B1%8D%E0%B0%9E%E...