I don't think uBlock can or does block it, here's the issue that tracks the research into it: https://github.com/uBlockOrigin/uAssets/issues/5184 as well as an attempt to block them here: https://github.com/instance01/Twitch-HLS-AdBlock

You may be ad-free for another reason such as a grandfathered twitch prime that will expire when it next renews, twitch turbo, or a subscription to a channel that opts-in to the ad-free sub benefit. You might also just be in a region or demographic where ads aren't being bought, so you wouldn't get any then.