That's something Twitch has been doing to everyone using any adblocker, not just with AdguardHonestly, I watch Twitch streams nightly for years. I've never seen an ad on Twitch with Adguard for Windows installed.
What I have noticed, for some reason my streams get limited in video quality with the adguard blocking on Twitch. If I stop blocking Twitch, the stream quality goes back up. I don't mean auto, I mean, it will only let me select 480p max.