It is likely that you have something in between that strips the IDs for some reason.
I’m not sure if this has to do with the Oxygen builder but I need to perform some tests with it.
To be sure, try deactivating all of your plugins except Joli TOC and see if you get the IDs or not.
Also if you want to debug, you can try this filter and see what comes out (it will show raw data on your front end):
add_filter('joli_toc_headings', 'filter_headings', 10, 2);
// Dumps the headings
function filter_headings( $headings ){
var_dump($headings);
return $headings;
}
Each item should have an id if it has been correctly processed. If not then something strange is going on.
And if it does have an id but it is not showing inside the <H2/3/4/5/6> tags in the final output then something inteferes in the process.