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I feel confused because the behavior of these functions is as I expected. Here is my code:

(defpackage :all-your-base
(:use :cl)
(:export :rebase))
(in-package :all-your-base)
(defun rebase (list-digits in-base out-base)
(let ((decimal (to-decimal list-digits in-base)))
(decimal-to-base decimal out-base nil)))
;; convert a list from in-base to decimal base
(defun to-decimal (list-digits in-base)
(reduce
(lambda (acc x) (+ (* acc in-base) x))
list-digits
:initial-value 0))
;; convert a decimal number to an out-base list
(defun decimal-to-base (decimal out-base acc)
(if (<= decimal (1- out-base))
(cons decimal acc)
(let* ((quotient (floor decimal out-base))
(remainder (mod decimal out-base)))
(decimal-to-base quotient out-base (cons remainder acc)))))

Have you tested your code against the test suite locally? See the documentation at Testing on the Common Lisp track | Exercism's Docs. Do you get any error messages running the tests locally? If so, what do they say?