There isn't a 1-1 of ascii character to base64 character is the thing. 3 ascii character = 4 base64 characters, so changing one of the input changes 1 or 2 of the output.
Trivially, there is a 1:1-correspondence between the set of all possible ASCII strings and the image of that set under Base64 encoding, because otherwise it wouldn't be an encoding anymore :)
This doesn't guarantee that a Base64-encoded string can't be identical to a substring of a different Base64-encoded string. And for arbitrarily short substring length (bit), this doesn't make sense either.
So we rely on some definitions and minimal data structures such as bytes.
What I meant was that AFAIK, the ASCII character set might have an image under Base64 encoding that allows for prefix-free encoding given a minimum read length.