It’s your site’s custom popup JS that’s to blame. And it can be reproduced in both Chrome & Edge (which have the same underlying engine, by the way — it’s nearly impossible for there to be actual differences there). The only reason it doesn’t happen as often in Firefox and Safari is your code works differently in those browsers.
The root cause is that you’re loading form 5760 with the initial page, and you’re also reloading form 5760 (MktoForms2.loadForm()) from Marketo whenever somebody clicks the button! This isn’t the right way to do it: you should load and render the form only once, then show/hide the popup on demand. It’s a faster user experience and also avoids any duplication.
The above happens in all browsers, every time. Any cross-browser/cross-session differences depend on whether the old <form> element is completely deleted from the DOM before a new <form> element is injected. If the old one is gone then the problem appears to not be present, but it’s still there: you just got lucky and the old one was removed first. And that’s the direct cause, which is that you have a race condition where the old form may or may not be removed in time.
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