ArtRage for iOS 2.2.1 should now be available from iTunes. This update includes various bug fixes for problems in 2.2.0 and iOS10:

A fix for the layer opacity slider not updating the layer visually.
A fix for tool blend modes causing problems with the Tool Settings Panel.
A fix for the sampler failing to trigger sample in some combinations of screen orientation / canvas size.
Sundry fixes for problems running on the iOS11 beta.
Fixes for misc issues that came in from devices with reporting turned on.

The App Store now also allows users to install older versions of existing apps, it appears the issue was temporary, so if you are not using iOS10 you should still be able to download an older version of the iPad app.