Apple tv

• There is the Apple TV app Channel for Acorn, which is hosted by Apple. It requires subscribing within the Apple TV app, as well as viewing inside the Apple TV app.
• There is the separate Acorn app, which requires a different type of subscription, either through Acorn directly, or through Apple (on Apple devices). This subscription is not valid for the Apple TV app Channel for Acorn.
• The separate Acorn app on third party devices may not recognize subscriptions on your Apple ID account. Then subscribe with Acorn directly.

Choose one and stick with it.

“An Acorn subscription through Apple TV” is a bit ambiguous, as it can relate to either one of these types off subscriptions, and these subscriptions are not exchangeable.

Subscribe to Apple TV channels in the Apple TV app - Apple Support

Acorn TV app for iPhone, iPad or Apple TV box - App Store preview - Apple

Apple TV Channels FAQ: Services, pricing, availability, and everything else you need to know - Macworld

Acorn is not an Apple product, contact the provider.

The Apple TV+ channel contains only items produced by Apple. It does not cover other streaming services - Disney+ for example - you have to subscribe to those separately.

I think your first option would be to contact Acorn who should be able to advise you. If you can't get any sense out of them, try https://reportaproblem.apple.com/?s=6