Because iPad browsing is becoming more similar to web, Safari version 13.0 and above uses a 'Desktop-class' experience by default, which categorizes your browser's User Agent as a desktop device. This has impacted a wide array of apps that collect platform data, including Heap.
Unfortunately, we do not have a workaround for effectively identifying and recategorizing these sessions. See the official Webkit update for more details.
Note that if the user loads or reloads a webpage while Safari is at one-third size, it will be categorized as a mobile User Agent, since the mobile layout is better suited to the smaller viewport.