This is a side effect of a feature we recently added. To get around the issue, you'll need to do the following:
Go to the forms page and click on the Login tab.
Scroll to the bottom of the page and click "Build a custom login form."
Click "Test login page" at the top of the page.
Enter your login page URL or the word "login" if you're using the default Memberstack signup modals.
Scroll back to the bottom of the page and select either "Pop-up modal" or "Build a custom login form."