This is result of jquery conflict with any third party plugin. to figure that out , deactivate all other plugins except this and woocommerce and activate each one by one and test. once you find the conflicting plugin, you can deactivate that plugin for some time whenever you want to make changes related to checkout fields. once changes are done , you can reactivate that plugin. it won�t affect the frontend functionality.
This usually happens when some third party plugin loads their js file on all admin pages which is wrong in their part. they should only load their js file on their pages or on pages where it is required.
To quickly find out the conflicting plugin , just look at the page source of our plugin’s admin page and just take note of plugin which are loading their js file that page. see this.