Starting with iOS 13 and iPadOS 13, Apple has made a change in the installation process that doesn't allow us to know which kind of device installed the app and therefore doesn't allow us to provide statistics on device types and versions which installed each of your apps.
Previously, when the app was being installed, iOS called us with a user-agent containing the device model and iOS version like
itunesstored/1.0 iOS/9.3.5 model/iPhone8,1 hwp/s8003 build/13G36 (6; dt:141) or
com.apple.appstored/1.0 iOS/10.0 model/iPad3,4 hwp/s5l8955x build/14A5261v (5; dt:83), which allowed us to make some nice stats on the app installations and provide that information on your dashboard.
On iOS 13+, the user-agent sent looks like
appstored/1 CFNetwork/1082.2 Darwin/19.0.0, which doesn't provide information on the device model at all, and very little on the iOS version (CFNetwork version can be, but might not be, specific to a version of iOS).
Therefore, installations on iOS 13+ will show the device as "iPhone/iPad/iPod on iOS/iPadOS 13" on your dashboard and hopefully we can guess the iOS version from the CFNetwork version, but that might not always be very reliable.
Eventhough it could be a move from Apple for more privacy, but the previous user-agents didn't share any personnal/identifiable information, it could also be a bug and might be fixed on their side in a future update!