Is there a logical explanation why 非得不 and 非得 means must? 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?
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.

References:

1. “非……不可”句式在对外汉语教学中的研究

2. 无可无不可的“不可”

3. 肯定、否定、否定之否定