Our development team had a look at this and here’s what they found:
When inserting a time tracking record into QBO, a TimeTrackingStartTime and TimeTrackingEndTime must both exist. If a value for both fields does not exist, both values are set to null.
In M:C, the record would not be posted to QBO until a save or send to desktop action was used. In M:N, the record is being posted immediately upon insertion, causing QBO to clear both values, as you of course do not yet have an end time.
You could try inserting a temporary value into the TimeTrackingEndTime field and overwrite this value once the stop button is selected with the actual end time. I’m not sure what effect this will have on your accounting however.
Another option would be to create a custom field to temporarily store the start time until the entry has been completed. You can then enter both the start and end times when the stop button is pressed.
Please post back and let me know whether this works for you.
Edit: This is only a requirement for QBO. Updated my post to reflect this.