I have my settings to:
For each article in a feed, show : Full text
Every post I have uses <!--more--> as you can see http://osxhelp.com/
All posts on the main page are truncated where I have specified them to be.
Any RSS feed, whether atom, rss, or rss 2, do not truncate where they should.
The code:
<description><?php the_content_rss('', 0, '', get_option('rss_excerpt_length')) ?></description>
This is rather mysterious to me, it calls the_content_rss(), which really does not do very much, at least, I do not see anywhere in that function that looks for the pattern <!--more-->
However, the function does nest another call to get_the_content(), which I can not seem to mentally parse out what is it up to. I see the match:
if ( preg_match('/<!--more(.*?)?-->/', $content, $matches) ) {
and in that it stuffs an array of $matches, which has the <!--more--> as item 0 and nothing as item 1 in the array. I see no way at all that this can properly work. I do not know the code well enough to confirm this is a bug.