This is what is writing in the documentation:

isbn10¶
type: boolean
If this required option is set to true the constraint will check if the code is a valid ISBN-10 code.

isbn13¶
type: boolean
If this required option is set to true the constraint will check if the code is a valid ISBN-13 code.

Here is the code:

if (10 === $valueLength && null !== $constraint->isbn10) {
...
} elseif (13 === $valueLength && null !== $constraint->isbn13) {
...
} else {
...
}

The problem is when I set to false the isbn10 and to true the isbn13 parameters, I get the bothIsbnMessage instead of the isbn13Message.

I suggest to change the code like this:

if (10 === $valueLength && $constraint->isbn10) {
...
} elseif (13 === $valueLength && $constraint->isbn13) {
...
} else {
...
}