Custom code issue

This past weekend, a customer reached out because they could not access the "cart" feature on our website after adding items to their cart. 

Instead of looking like the first screenshot (with the cart button in the top right), it looked like the second screenshot (cart button was hidden under the header, barely visible, just a tiny black stripe).

My assistant went to the website and tried to place an order, but encountered the same issue. After about 30 minutes of clicking around, the issue finally resolved itself and the website worked properly again. 

I reached out to Squarespace Support and told them about this issue. They went into the backend and could see that the problem is being caused by Custom Code in the header of the website. 

Squarespace cannot advise or help me with this issue, since it's being caused by code not created by Squarespace. 
The code is from Flodesk, our email software.
I've used Flodesk forms for years, with no issues. I recently updated my homepage header form in late Feb, 2024. 

Is it possible there is a problem with the code in the form? Do I simply need to remove the form/code from my website?

I'm trying to figure out where the issue is coming from - is it Flodesk or is it  Squarespace? 

We're scheduled to launch our biggest sale of the season next week and need to have this figured out ASAP.  

I appreciate ANY insight or advice you can give me. 

Below, I've copied and pasted the response from Squarespace:
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Regarding the issue with the add to cart icon, I have just tested your site on my end, and I am also seeing this issue. However this issue is being caused by custom code that you have added to the site. I can see this because when I enter your site in Safe mode, which disables scripts, the header disappears, which means that you've added the header by way of custom code and the add to cart button is then hidden by it. 
 
You can see this as well by logging into Squarespace, and clicking on your site in your dashboard, then adding /safe to the end of your site's url while you are in the back end of your site. For more information, see our guide below:
 
https://support.squarespace.com/hc/en-us/articles/205815908-Using-code-injection#toc-disable-scripts-in-preview
 
While you're welcome to use CSS, we aren't able to offer troubleshooting support for advanced modifications like custom code. We're unable to offer assistance with adding, editing or troubleshooting for such modifications, due to their complex nature.
 
Custom code modifications may cause unwanted results when interacting with our platform, templates, or updates we release. As per the guide below, we’re unable to advise on or troubleshoot code related issues or changes to the site
https://support.squarespace.com/hc/en-us/articles/205815928-Adding-custom-HTML-CSS-and-JavaScript

Our Squarespace forum is another great resource:
https://forum.squarespace.com/

Hi @smallbusinesslorijhern Sorry to hear that. As described by the Squaresapce support team, the issue is caused by adding third-party code. So there is nothing Squarespace can do.

4 hours ago, smallbusinesslorijhern said:

My assistant went to the website and tried to place an order, but encountered the same issue. After about 30 minutes of clicking around, the issue finally resolved itself and the website worked properly again. 

Do you mean the issue is resolved? So there is no issue with your website currently?