That's just assign wearing off.
If she could negate assign with haki, it straight up wouldn't work on her. She wouldn't need to use physical strength to lift the tower, she simply wouldn't be attached to the tower anymore.
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Awakened abilities are simply harder to negate with haki. For example:
- Luffy turning Kaido into rubber
- Law being able to use shock wille on big mom, but not teleportation
- Kid being able to assign Big Mom
That's not to say its impossible. If anyone could negate awakened abilities, it would be shanks or mihawk. However at the moment, we've yet to see anyone who can actually do it.