Curious behaviour: Mid(string, -3, 7)

I made an error in my code and suddenly I used a negative start value in a Mid function.
And surprisingly everything worked with negative numbers:

dim t as string = "exemple" dim t1 as string = Mid(t, -3, 6)
and t1 = “ex”

If we use negative numbers and zero this is logic, but not intuitive.
I think that at least should be commented in the LR

Except -3 is meant to be the START and it makes no sense in this context.