We've tracked the issue down and in order to fix it, we'll need to do some work on our end and update the app itself. When an iPad starts to run out of available space, it goes looking for things it can delete in order to free up space. Usually, this is in the form of temporary files. Unfortunately for us, one of the things it deletes is the "map" the app uses to identify where your sessions are stored. 

In the meantime, I suggest you delete the app and redownload it to recover the extra space from the sessions you've lost. You'll also want to make sure you have at least 2 GB free of available storage to keep the "map" safe.

(iOS 10) You can check your available space by going to the Settings app > General > Storage & iCloud Usage and your free space will be listed next to Available under the Storage heading.
(iOS 11) You can check your used space by going to the Settings app > General > iPhone/iPad Storage and your used space will be listed at the top, above the usage breakdown graph.