must not/mustn't

[Pooh]

I am not sure about the meaning of "must not".
Look at the two sentences, I think the meanings of "must not" in both sentences sound slightly different to me.

1. I must not have been thinking straight when I did that.
2. News must not be exaggerated or understated.

If I paraphrase the two sentences above, they could be said like;

1. I am sure I wasn't thinking straight when I did that.
2. Don't make any news exaggerated nor understated.

Am I right?

Also, I've heard that native English speakers don't use the contracted form of "must not" anymore.
Is that true? Can't I say "mustn't" instead of "must not" for these two sentences noted above?

Thank you in advance.

[Mordant]

You have paraphrased them very well. Those uses are not the same. "Mustn't" sounds rather dated.