Tested on v6.0.0, Linux x86_64 (musl):

(import (scheme base))
(define port (open-input-bytevector #u8(1 2 3)))
(peek-u8 port) ; 0, should be 1
(read-u8 port) ; 1, is correct
(peek-u8 port) ; 0, should be 2

-- Peter McGoron