Yes it does mean what you say, to an older generations ear. It is a well used sentence structure to many native English speakers. You will get lots of disagreement on the answers. "you must not be surprised about that" = do not be surprised. two different ways to say the same thing. You should see (must+not)+"be as one compound word although it isn't. So it is also the same as "you shouldn't be surprised" Perhaps an easier example is a parent to a child "you must not run into the road" = a command/imperative. You can also swap the imperative command with anything else, like being surprised. 1."you must not run into the road" 2."you must not be surprised" 3. "you must not touch that" -->. warning a child or someone to not touch live electric wires/dangerous liquids, acids, guns !! 4 "you must not be happy about that" or "you are unhappy about that 5 "you must not be sad" or "you must be happy" 6 "you must not be very pleased about that" = "you must be very displeased about that" the best way to view it is by rewriting the sentence in the opposite way or with the opposite word. or by negating the word like this. "you must not be happy with that" or rewrite like this "you must be unhappy with that" no need in most cases to try to rewrite with can't etc just negate the word like the example above.

Saying 'must not be easy' does mean 'is difficult'. I disagree that using 'must not' is odd. To me, it is fine - but perhaps it is a little old-fashioned.

"It must not be easy" meaning "it must be difficult"-- Americans would consider this a native-sounding phrase. I'd say it's more common in the US than "it can't be easy"--but both will be clearly intelligible and recognized as correct-- :)

I agree with Karen. But "must" doesn't sound strange to my American ears. But to answer your question, the phrase "it must not be easy to ____" does mean that it is difficult. difficult = not easy. The confusion is in the word Must, which normally means an obligation. I must work tomorrow. However, I checked the dictionary and read all the many things must can mean and I found these: used as an auxiliary to indicate the probable correctness of a statement used as an auxiliary to express conviction or certainty on the part of the speaker these two definitions show how must has other meanings and implications. The other issue is the negation, which with must always feels odd. Because of this, I agree with Karen above that can't works better, or isn't.

No, not if you mean the opposite..