Chinese handwriting on Facebook can't be entered

iOS 11.2.5(15D60), Facebook 159.0.0.48.97 (91994325)

This happens when I try to reply some comment on FB.

I use the Chinese handwriting ...

I handwrite some character, that character would get highlighted.

No matter what I do the character would get highlighted and I cannot select it and move to the next space. The only way to move out of the position is to select some other suggested character which isn't what I want. So in a way the whole Chinese handwriting becomes useless to me.

It's the strangest thing. It seems only happens in FB reply but not in other apps. I posted the same question on FB support.

I tried remove FB and the Chinese handwriting keyboard and reinstalled, to no avail. I rebooted the iPhone but to no avail either. I tried another iPhone (though I am not sure the versions of the iOS or FB), no such problem.

It seems like when I write on FB reply, the character is highlighted but can't be selected for input. However if I choose any other suggested characters then I can select and input.

I don't know if I am even able to describe the problem clearly.