I have a on my checkout form, and somehow Chrome thinks that this field is a credit card expiration month.
Add autocomplete="nope", to your input tag if you don't want it to auto-populate with anything. The browser will try to process it and since nope is not an acceptable option for it the browser will give up on processing it.
See - https://developer.mozilla.org/en-US/docs/Web/Security/Securing_your_site/Turning_off_form_autocompletion#Disabling_autocompletion.