Why are the email signup forms on this website opening in a new tab if submitted with errors?

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I have three Mailchimp forms on the website that I am working on. For some reason, on two of them, if you click “Subscribe” with all fields blank, it opens a new tab with the form and error messages saying “There are errors below” and “Please enter a value”.

On the other one, it works properly, if you click “Subscribe” with all fields blank, it just displays “This field is required” underneath each field.

I have checked the code for all of these forms and they are all the same, I can’t see anything that would cause the error message to open in a new tab. Has anyone else experienced this issue or have a fix?

Here are the HTML codes, maybe someone can spot something that I can’t:

This one does not work:

This one does work:

This one does not work:

Some screenshots, here is what it should look like:

Here is what it should not look like:

The live website is

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