Why do I have a erratic iSO adjust after manually setting shutter speed?

[ExitAisle]

erratic ISO adjust after manually setting shutter speed

When I change the shutter speed in Lumia Camera lens on my 1020, sometimes I see red lines under the iso and shutter speed to indicate they are adjusting in tandem; the picture using a higher shutter speed is properly exposed... sometimes his doesn't happen. sometimes when I adjust the shutter speed the ISO does not adjust and the picture is either blown out white or solid black... this happens sporadically and I can't see a pattern... Assuming this is operator error or something I don't understand, I wonder if there is a situation in which the ISO is NOT supposed to work in tandem with shutter speed to insure correct exposure? I feel a need to use higher shutter speeds because it's so hard to hold the camera steady.

 

[bijak_riyandi]

The red line means the setting you're using is not recommended by the app's algorithm, but it won't block you from using it anyway...

EDIT: I remember a Nokia video on YouTube during the early days of the 1020 when they used the manual setting when they were taking pictures in an aquarium.
That's how I understand the red line...