It's explained that Rachael is a shop-floor floor demo model, allowed to be on Earth presumably under the strictest of conditions, one of which appears to be that she stays under Tyrell's direct control.
BRYANT: Looks like Holden found out that the V-K machine doesn't work on the Nexus 6. The Tyrell Corporation has a demo model so go check it out. If the machine fails, we're in deep trouble
When she leaves, she's no different from any other rogue Replicant.
DECKARD: Three. There's three to go.
BRYANT: Huh?
DECKARD: There's three left.
BRYANT: Four. That tit job you vee kayed at the Tyrell Corp... disappeared. She didn't even know she was skin till you put the machine on her. Some kinda brain plant, says Tyrell.
Note that the implication is that Tyrell reported her missing, so it's unlikely he'd be willing to risk his corporation for her.
This is confirmed in the film's official novelisation. Tyrell evidently reported her for destruction. Once she knew her true nature, she was useless to him.
"Your arithmetic is lousy," said Deckard. "Just two left.”
"There’s a new one loose,” said Bryant. "It belonged to Tyrell himself. He called to say it was ready for the scrap heap. But when we went to pick it up, it had gone on the lam.”