[worker767424]

It's possible that the client blindly fetches a mapping from keyword to ads, saw the keyword client-side, then requested the ad.

[Drakim]

You'd indirectly reveal what those keywords are to Meta by which ads are being requested. If an ad for a sex toy is being requested, it's pretty obvious what the two parties are talking about.

[atoav]

Still information leaks the encrypted channel and the trust is broken.