There is a bug with slider widget :
create a slider and link it to a stream, save , it’s ok
Come back to editor and move the widget to any place and save , you will lost data stream of all sliders 
same if you change anything , I tried to change color label , same issue
My guess is you expecting the values to be saved and viewed in Template setup Dashboard instead of Device dashboard. Am I right?
Also, the device should be online when changing Slider position.
Wrong, please re-read what I said, the problem is in the editor.
Only sliders are impacted,
the device is online, and sliders work fine until I try to edit any widget on the dashboard.
1/ go into editor mode and edit the slider widget
2/as you can see, datastream is lost
3/choose a source and save
4/return to the editor, and edit slider again
datastream source is lost again
If you have more than one sliders, all sliders are disconnected from their original stream source
Thank you for your report!
Our developers are already fixing this issue.
The clarification by steps 2 and 4 is extra helpful.
Thanks for you report!
You are welcome