Why does iFrame show incorrectly on return?

(1) Go to https://realestateschoolchicago.com/real-estate-broker-classes/class-schedules/
(2) Click on any "Register" button.
(3) Hit the "back" button on your browser.
(4) Note that the button text is no longer visible; instead, at least for me, its showing the word "Email"

It's a WordPress site, iFraming Shopify "Buy Now" buttons.  How can I make the button appear as it did before clicking?  Thanks, Experts!!  :)
Harrison WalkerAsked:
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William FulksSystems Analyst & WebmasterCommented:
It worked fine for me, but it was a bit slow in pulling up those buttons. You might be better off using a single graphic file.
Lucas BishopClick TrackerCommented:
I saw the error using Chrome. Looks like an error/bug with the plugin you're using: https://apps.shopify.com/shopify-widgets

This is a plugin created by Shopify, so they should offer direct support via their email address: support@shopify.com

The documentation is very thin, so there's not much available in regards to existing troubleshooting info to dig into.
Harrison WalkerAuthor Commented:
Thanks, William and Lucas.   I didn't check in other browsers, but I use Chrome, as does the majority of our user base.  And I went to Shopify, but they said that it's not their issue, since it's on an external site.  I suppose that I could look into creating a single graphic file and figuring out what link to use for each button, but I'd rather use their iFrame button if possible.  Anyone else got an idea?  Thanks so much!! ~ Scott
William FulksSystems Analyst & WebmasterCommented:
You could drop some code in there to force the browser to not cache the page so it will do a reload every time it displays. That may fix your issue.

Prevent caching - http://www.htmlgoodies.com/beyond/reference/article.php/3472881

Force page refresh with Back button - http://www.mrc-productivity.com/techblog/?p=1235

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