This question is an exact duplicate of:
Something funky has been happening since the Stack Exchange app was updated on my iPhone: every time I write a comment, I get a success message, but the question goes blank and can’t interact with it.
Screenshots of the problem:
The solution/workaround to the problem is simple: exit the question (the top bar is still there) and get back in, and everything will load fine (including the comment I wrote).
Is that something happening to more people, or is it only my iPhone? (6S for more detail.)