When in darktable, look at the history stack on the left hand panel. Starting from the top click om each entry until in turn , in other words go back through the history until you find the module that is causing the effect.
Now look at that module on the right hand panel to determine what is causing the issue. If possible, adjust the module settings to get the image the way you want it
You can then save this setting as a preset if you wish and also have it auto applied to other images if you wish.
Can I suggest you look on Youtube for Bruce Williams. He has 60+ videos on DT going from the basics onwards, well worth watching to get you started