I wanted to add some white space between the TOC text and the leader symbols in my TOC. Here is the default TOC when I don't use whitespace:

I tried the following Expression:

= Fields.group1 + "            "

Here is the result - the TOC text of the first level was displaced as if the added whitespace is before the text:

The workaround I found is to add one LeaderSymbol at the end of the whitespace string:

= Fields.group1 + "            ."

Still, it seems that there is whitespace trimming that is not correctly implemented for the TOC section.