Scenario's shouldn't loop unless you have the 'Repeat' option set to 'yes'!! Its top right of the screen under the scenario name and description. Both options should be 'No' for your lamp situtation.
Normally the scenarios just run through the sequence of 'Actions' and then stop when they get to the end of the sequence. Then they will only start again if the start condition occurs again. There should be no need for the stop command.
If the Repeat option is not selected then I am puzzled. In that case could you post a screen shot of MyHome Suite.
Stop panel is used to interrupt a already running scenario. eg when you want to start another scenario which has some of the same items in the action list as an already running scenario. eg blinds with staggered closing and then another scenario starts to open them. If you do not stop the first scenario then both scenarios could run at the same time causing confusing results. FYI Up to 5 scenarios can run simultaneously.
For debugging and testing you can see what is running by accessing the MH202 via your browser and going to the scenarios tab. There all scenarios are shown with the running ones highlighted. You can start and stop them too.
For testing set the times around the time you are working on it and replace sunset with an hour. Then there is no need to wait for sunset or midnight to get a result
So I'm finally home to test your instructions.
I set the F428 plugged with my Eltako Sunset sensor with contact 1. The Eltako sensor shows a 'dark' status'. The Led on the channel 2 is not turned on.
On the MyHomeUp app I'm t rying to search the F428 to create a status display, but I cannot find any F428.