Does iPhone support the use of Northern Khmer in Thai script? I would count an interface in Thai as support.
The reason I ask is that I tried entering the word กฺี <U+0E01 THAI CHARACTER KO KAI, U+0E3A THAI CHARACTER PHINTHU, U+0E35 THAI CHARACTER SARA II> 'he' and got a dotted circle. I also got a dotted circle for the alternative spelling <U+0E01, U+0E35, U+0E3A>. This might be an application issue. The application I was using was Line. Richard.