It turned out to be that the page depended on some HTML files in Prod which were saved in Salesforce Classic under Documents. Documents don't get copied over, and so those dependencies were broken. Those HTML files were loading jQuery as well as some other things needed.
I went into Prod, downloaded them, and had to go into Salesforce Classic to upload them. ...
It's possible that you need to set up permissions for your component to load the external jQuery library. Try going to Setup > CSP Trusted Sites and make sure you have an entry for https://code.jquery.com/.
If there is no entry, click on New Trusted Site and add https://code.jquery.com/. Make sure you check the checkbox for Allow site for connect-src.
In Spring'21 Salesforce release,
Get More Characters for Field-Level Help Text
Customers can view the field-level help text by hovering over the Info icon. And with the character limit expanded from 255 to 510, you can now provide more detailed help information. This increase applies to standard and custom fields on detail and edit pages.
This change ...