Please add screenshots for me to better understand what problem you are seeing.
There's also other features in the app that can cause the duration to be smaller than the difference between start and end, e.g. the duration adjustment tag or the break feature that can be set up in the settings. Or maybe you use the idle detection that pauses the timer once you leave your PC for a while...
If you are certain that there is a bug, I would highly appreciate some details on how to reproduce this problem.