Why can I not change microphone inputs on Garageband for iPad?

I am using an 11 inch M2 iPad Pro with the latest version of GarageBand and some Apple earbuds plugged in through the Apple USB-C to 3.5mm adapter.

I want to record a multitrack demo in Garageband using the internal microphone.

I record one track, then plug in my headphones so I can listen along while I record the second track.

The problem is, Garageband then defaults to my headphones built-in mic, which sounds terrible. However, despite the iPad being sold as a content creation device, I cannot perform the simple task of selecting which microphone I want to use. Why is this not possible?