Hey Jeremy, we were just talking about this as a team again this morning. Because of the way that custom LTI launches are handled in the Canvas mobile app, users are required to have an active web session in order to view Arc embeds. This is the case with other LTI tools that do custom launches as well. The mobile app doesn't hold on to an active web session indefinitely for security reasons - an active web session would mean giving indefinite full access to a user without requiring them to log in periodically, whereas the current approach gives them limited access via the mobile app indefinitely.
We're working with the Canvas Mobile app and Platform teams on possible solutions, it's definitely a bit of unnecessary friction we'd love to remove as soon as possible. Some customers wanting to avoid this on iPads are using Canvas in a browser instead of the mobile app which avoids the LTI authentication issues.