OK, I try what you suggested (I had over 30 different UI Policies and Scripts), and have goot news and bad news.  The good news is that I have seemed to locate the offending code.  It is a Catalog UI Policy in the same Variable Set that is having issues (so that seems to make sense). 

But the bad news is that particular Policy isn't even touching that manual_class_id checkbox variable.  So I don't understand why it would be affecting it.

Here is what it looks like:

As a double-check, I went back and deactivated every single Catalog UI Policy and Catalog Client Script in this Variable Set, and just activate that one, and the error shows up (I thought it might be interacting with other ones in the same Variable Set, but that disproved that theory).

Any idea of why that would affect why the check is not showing up in the manual_class_id box on the RITM?