OK, not dealing with the styles at all, let's address the TOC field.

Delete your Table of Contents and go to insert a new one. This time, instead of choosing one of the options you are given, click on the Insert Table of Contents at the bottom. This brings up the dialog box. Select the number of levels you want and then click on the options button.

It will show you all styles that are captured for your TOC with their levels. Go into the box for the level for the styles you do not want captured and delete the level numbers. (I put the insertion point after the level number and backspace.)

OK your way out.

If you press Alt-F9 to reveal codes you will see a field something like:
{ TOC \o "1-1" \h \z \t "Heading 2,2,Heading 3,3,Heading 4,4,Heading 5,5,Heading 6,6" }

instead of one like:
{ TOC \o "1-3" \h \z \u }

Press Alt-F9 again before you forget!

The guide to these "switches" in the field can be found at
Field Codes: TOC

I suspect the crucial one is the \u switch.