Confirm Password Validation Not Working

I have the same issue, I believe it is because I added modal login/register form to all pages in my theme. The register modal has a field with id="password" I think the validator is checking this value rather than the desired input.

I have not fixed/tested this theory yet but it would make sense. Double check to ensure you do not have an additional id="password" on these pages

Should check your template again.

1) Make sure we didn't need change id of password element. Because the confirmation password based on it:

vendor/magento/module-customer/view/frontend/templates/form/register.phtml

data-validate="{required:true, equalTo:'#password'}"

2) Should try to debug the Js: https://developers.google.com/web/tools/chrome-devtools/javascript/. My Magento version is 2.2.0 and the Js validation is jquery.validate.js. In your Browser (I used Chrome), open Sources Tab and debug.