I've bought an iPhone X about 3 months ago, 1 month later I started facing a problem where some Arabic letters and words are being cut and not shown in the messages in Messenger, Facebook, Line and even the Messages app. While the same messages are shown normally if I come back later.
It happens from time to time, but recently it became so much worse. The English text is shown without any problem
Currently I'm running iOS 11.2.6
I'm not sure but have you tried to put the global language of the iPhone in Arabic? Applications can change behavior.