Your two images look essentially identical. Here is a map of the difference once one is scaled to match the other:
The white is the pixels where the difference in level is 3 or more (difference along contrasting edges is due to rescaling).
As far as I can tell the purple bands are in both images, and it indeed looks like banding:
A smartphone is not a good ...
What you are probably seeing is the difference between how your display devices scale down the larger image and how they display the smaller image when you look at it on Instagram.
Viewed at the same size, there's no significant difference between the two images.