"multilevel-toc-expandable" is not defined in $:/core/macros/toc, so I assume it is a custom macro.
There are several custom versions of TOC macros that have been written by others.
Can you post the source code for this macro (or at least a link to where you got it)?
Note that the "exclude:" parameter is actually intended for internal use.
The value of exclude is dynamically updated as the TOC recursion occurs,
in order to prevent "loops" caused by revisiting a tiddler that has already
been displayed in a given branch of the tree.
-e