In the documentation for the between function called when using ?between, the function is described as

a shortcut for x >= left & x <= right

However, between handles NAs in a substantially different way than the quoted base R code would.

For example, if we called

2 >= 1 & 2 <= NA

we would receive an NA.

However, if we call the following

between(2, 1, NA)

we receive FALSE as the return value.

I think that a return value of NA makes more sense than a return value of FALSE here, but, if FALSE is going to be returned, then it seems like this discrepancy with the base R code ought to be clearly documented in the help for the function.