iPad brightness fluctuates while using Freeform + Pencil

I have tried restarting my iPad and it's running the latest version of iPadOS. I've tried to replicate the problem with and without the iPad case on and it occurs in both cases.

When left alone, iPad brightness is constant - when using a pencil to make notes in Freeform the brightness dims, typically in three distinct stages. When interaction stops then the brightness eventually returns.

As Apple doesn't provide access to the device internal temperature sensors I can't speak to whether it's an overheating issue, but I am using an iPad case and, when I remove the iPad from the case it is quite warm. As above, I've replicated the experience with the iPad on its own without a case.