We need it to trigger on reborn. Using it in BG. Triggers on others, but not reborn.
Because your minion doesn’t lose reborn
Your minion with reborn dies
A new minion without reborn is summoned
They are 2 separate minions
A deathrattle minion doesn’t lose its deathrattle when it dies 
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Maybe it was broken previously and they corrected it then… it was triggering on reborn and that is a “keyword”.
A minion will lose Reborn is when it is attacked by a Sin’dorei Straight Shot, which removes Reborn and Taunt from its target. So that is one case where Implant Subject would correctly trigger on Reborn.
But as @Derkan said, if a Reborn minion dies normally, that does not count as losing Reborn. The Reborn minion is destroyed, and a new one without Reborn is summoned.
The new minion never had Reborn, so it was not “lost”.