The comments were spot on, the library only supports a maximum of 54 bytes. This test confirms it. EQUAL=false begins at 54 bytes. Thank you to the helpers in the comments.

@Test
fun test() {
    for (n in 100 downTo 0) {
        val secureRandom = SecureRandom()
        val bytes = ByteArray(n)
        secureRandom.nextBytes(bytes)
        val password = Base64.getUrlEncoder().withoutPadding().encodeToString(bytes)

        val passwordHash = BCrypt.hashpw(password, BCrypt.gensalt())
        val modified = password.dropLast(1)

        val equal = BCrypt.checkpw(password, passwordHash) && BCrypt.checkpw(modified, passwordHash)
        println("BYTES=$n EQUAL=$equal")
    }
}