FM 2020 Changes to Responsive HTML 5?

Hello. I've spent a ton of time getting a .css that matches a book with over 35 chapters. We get the pdf ready for the printer, then publish the same content to HTML 5 for a software product. 

For the past number of years, this would happen fairly seamlessly. There would be cleanup of tables, but the overall formatting was good-to-go for the most part. However, when I used FM 2020, the output was radically different than it has been in the past -- the index isn't working (it's showing the Frame code used for the index markers), most of the styles in my custom .css aren't being applied (a few are ie., for body and headings), among quite a few other things. 

Previous builds have also only generated one .css, and I'd add my custom .css  tho all the files using regular expressions. Now it's auto-generating three different .css, including one that appears to be for Robohelp -- which I haven't seen in years.

-------------------------------

<link href="../../rhstylemapping.css" rel="stylesheet" type="text/css" />

<link href="../LLD.css" rel="stylesheet" type="text/css" />
<link href="../../editstyle.css" rel="stylesheet" type="text/css" />
<link rel="StyleSheet" href="../../responsive_html5_!OverrideStyle!/Styles.css" type="text/css" />

--------------------------------

I've tried both overrides and not overriding this, with the same result. 

Any help or information as to why this is happening would be most appreciated!

----------------------------------------------------------------