I haven't assigned GDTF modes to anything. The field is set to none. Also, as you can see in this picture, even fixture mode has no effect.
I store my custom symbols in a separate file that is under the "Favorites" tab and then import the symbol definition into my working file. I just built a symbol from scratch. When I place the symbol in space in the symbol file, the Num Channels field is available to edit (Screen Shot 2). When I open a new file and drop it in, I have the same issue.
When I start a new file, I use a template file that I've created. Could there be some deep buried setting that is causing the issue. I've also attached that file.
LD-CDS-V22.0.vwx