If you open the theme customiser by going to Online Store > Themes and clicking the 'Customise' button next to your theme. In the dropdown at the top, select Products > Default Product.
Then on the left, choose 'Product Pages', and on the right 'Stay on current page when item is added to cart'. Then save.
Hope that helps!
Hmm that’s an interesting one.
Have to checked to see if the setting is still checked? In case it’s being overwritten by something.