Well the fact that this is a common problem among the Facebook users, only when we use the iPhone and voice dictation, are you sure that it has nothing to do with Apple?
Moreover, today I was in a chat with customer care of rabbit app (this is a supermarket delivery service) and the representative's chat had the same issue with (obj).
so I asked him what is he writing on , and he replied Mac computer.
So as you can see this is a problem with many apps like Messenger, Facebook, Rabbit, and it only appears when we use Apple products.
now this is very frustrating, taking in mind that this is a 2 trillion company with high end technology.
and definitely has to do with Apple not with just Facebook.
there is a long discussion about this issue in Facebook Grec Apple community Group
and of course the answer that those applications were not developed by Apple, it's not accepted, we're not going to use only apples applications! obviously!