Keyboard shortcut: shift+tab in chrome does wrong thing
As a workaround, or just to check whether the problem is with Google Chrome or something else, install this extension.

To configure it to go to the last input, type in Shift + Tab in Shortcut Key and as the Action, click Browser Action > Inner Page Action > Focus on the previous input element.

You could also try Reset Browser Settings and then try going back in a form in Incognito Mode - &#8984;+Shift+N