The docs say

If no return is found and if the last statement is an expression, its value is returned. If the last statement is a loop control structure like a foreach or a while, the returned value is unspecified. The empty sub returns the empty list.

The last statement in your example is NOT an expression —it's an unless statement— so the value returned by your block is unspecified. That means you can't count on it being anything. I explained what it normally returns, but as you showed, that doesn't always hold.

So, you can't count on a block ending with an if statement unless statement resulting in something useful or predictable.