iOS 15.0 seems to have broken something with hearing aid streaming. Here is what I and an unknown reddit user found.
If you have Audio Routing set to Auto, your hearing aids might not stream audio. Set it to Always Hearing Devices and that should fix it. You can set it to Auto again after that, but if you turn off Bluetooth and back on again, it will be broken again. It will also break if you restart your device with it in Auto. This broke for me (and was fixed with the work-around) on both my iPad Pro 12.9” (1st gen, I think, 2015), and my iPhone 6S.
Settings > Accessibility > Hearing Devices > Audio Routing > Call Audio
(and also > Media Audio)