Bhagavad Gita, Chapter 14, Verse 6

Of these Sattva, from its stainlessness luminous and free from evil, binds, O sinless one, by attachment to happiness, and by attachment to knowledge.

Even the purest of the three forces named here is still described as real, genuine bondage, just a gentler kind.

Sattva brings real happiness and real knowledge, but Krishna doesn't let that comfort hide the fact that it still binds. Real honesty here refuses to treat the nicer mode as an exception.