Thanks Carl.
The JSON is being submitted in its entirety, confirmed by Fiddler. And it's actually being successfully saved to a SQL Server nvarchar(MAX) field too. I can validate that saved JSON.
I'm actually grabbing the JSON to convert directly from the SQL Server, and your comments / thoughts led me down the path of resolution.
Turns out that the JSON was being truncated prior to getting to the DeserializeJSON command, but it was the cfquery pull that was doing the truncating. The fix was to enable "long text retrieval (CLOB)" for this datasource in CF Admin. I'd never run into that before or even knew that this setting existed.
Thanks again for your comments!