Timestamp Source is behaving differently in case of offline mode and Online mode.
As per our understanding Timestamp Source should be captured on the basis of source(Mobile/Desktop) not on the basis of mode(Offline/Online).
As I came to know from internal team Field Service App is designed for only Offline Mode to captured the correct Timestamp Source. Means, if we use offline mode using Mobile App then Timestamp source will captured as "Mobile" else it will captured as "Desktop".
I would request product team to please look into this idea to meet the purpose of that field (Timestamp Source) to capture the Source.
We really required this feature for the customer for user adaptability and track the changes done either from desktop or Mobile.