Why does piping the output of $RANDOM affect the value?

Below is a script that I expect to generate the same pair of numbers three times:

#! /usr/bin/env bash
RANDOM=5
echo " first" $RANDOM
echo "second" $RANDOM

echo

RANDOM=5
echo " first" $RANDOM
echo "second" $RANDOM

echo

RANDOM=5
echo "??????" $RANDOM | cat
echo " first" $RANDOM

Instead, piping the output prevents the random seed from being respected. Why?

 first 18499
second 9909

 first 18499
second 9909

?????? 843
 first 18499

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Ultimately, I'd like to be able do something like this:

#! /usr/bin/env bash
n=3
for i in $( seq $n) ; do
    RANDOM=5
    echo $RANDOM | md5sum | cut -d' ' -f1
done

What modifications can I make to the latter script so that its output is the same each time I run it?