I have recently noticed that there is an outline around buttons in iOS. At first I thought that this was because of the setting “differentiate without color” but it remains even after I turned that setting off. Is this a new feature or some other setting I have turned on?
Go to Settings > Accessibility > Text Display & Size > Color filters, activate “Color Filters” then select “Red/Green Filter” and finally disable “Color Filters”. The problem seems to come from “Grayscale”; if you disable “Color Filters” with this choice of grayscale, the buttons remain highlighted.