If it was Brave itself, then it wouldn’t work in Private mode.
But all I know is TwitchAdSolutions pushed an update few hours ago, to fix the scriptlets they offer (which are hit and miss) because logged in users weren’t being able to watch streams/block (some) ads.
TwitchAdSolutions has two scriptlets and seems like both were having issues as well, so it was something Twitch changed that affected ‘adblocking’, it was happening to me, and updating the scriptlets fixed it.
The problem is when blocking the ad before the stream loads for logged in users, that’s what causing the issue, so only a third-party extension/adblocker is causing the issue like a most likely garbage adblocker like Adblocker Ultimate (at least use uBlock) or the Alternate Player extension.
My issue was caused because of having my scriptlets outdated in Brave and uBlock, so it wasn’t working properly, so you don’t know what those extensions are doing but they probably suffer the same issue scriptlets from TwicthAdSolutions were suffering before the update like 10 hours ago.
But Brave Team hasn’t updated lists regarding Twitch recently, Brave VANILLA doesn’t have those scriptlets included in their resources file by default, Brave Adblocker also hasn’t been updated in weeks which means no new features or anything in Yesterday’s update, so nothing should be a problem in Brave and people shouldn’t jump to conclusions and blame Brave for it.
In fact, I tested it in a new profile, without my custom rules, lists or scriptlets and even even without uBlock or anything else, just Brave Shields in Aggressive mode and Brave loaded the stream just fine, of course it showed the Xbox ad, but it loaded fine.