Please see the discussion in:
Having the analyst / field technicians enter their time spent is the primary mobile app use case for our customers.
I have since noticed that when adding a comment on an activity in the mobile app that it is recorded as a separate log comment activity and not as the user description of the chosen activity.
This is not the behavior of the standard web or REST API interfaces and should be addressed at the same time as the time spent. This prevents creating standard reports and event triggers.