This has been a problem since day one, and it's come up in the forums multiple times. People press Save Fit Data without first pressing Timer Controls > Save, then they wonder why they can't open their FIT file in monkeygraph (for example)
In general I think people still find Data Fields > Activity Data to be a bit confusing (although I think the UX has definitely improved since the beginning.) Tbf, I'm not sure how to make it more intuitive except for one suggestion below.
Is there a valid use case for being able to save the FIT file without "finalizing" it first, by pressing Timer Controls > Save? I don't think this can happen on a real device, except maybe in an extreme situation like an activity crash, unexpected reboot, or power down due to low battery.
If there isn't a valid use case, then it seems that Save Fit Data should be greyed out until Timer Controls > Save is pressed. Or perhaps Data Source > Save Fit Data and Data Field Timer > Save can be combined? (After all, Save is a single action on the device). i.e. get rid of Data Source > Save Fit Data, and just save the FIT file when the user presses Timer Controls > Save.
If there is a valid use case (like idk, maybe saving a copy of a prerecorded FIT file that was loaded), then at the very least, the simulator could display a warning if you press Data Source > Save Fit Data at a point where the resulting FIT file will be corrupt.
