I'm doing something a bit more complex for a plugin, but solving this riddle will solve my issue as well.
In hooking do_shortcode into the_content like so:
add_filter('the_content', 'show_shortcode');
function show_shortcode() {
echo do_shortcode('[html]Text[/html]');
}
...it echos/returns 1. Other shortcodes work as expected, but none of the ones associated with this plugin do. Switched wp_debug on to no avail.
What am I missing here?