I understand that 非 and 不 have negative meanings by themselves, and 得 means "must" by itself. So with a double negative, which means affirmative positive. So far so good.
But 非得 by itself is also "must". Single negative. Also positive. Why?
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非得 itself is always negative. The sentence contains only 非得 could be positive when there is a 2nd negative (most likely 不可) being omitted. This omission is common in colloquial language but not as common in written language or formal speeches.
Reference #1 below is a semantic analysis for the pattern 非..不可. It mentions the fact that 不可 can be omitted.
Reference #2 contains a lot of literature/publication citations where the 2nd negative of a double negative sentence is omitted.
In reference #3 the writer thinks the omission is grammatically wrong and is due to people not treating the language seriously.