The spoilers do work correctly in latest activity. Text content is removed from the snippet as expected. But the display of (attached) images here is actually a bit of a special case thing that we do specifically in latest activity / news feed items.
i.e. the text: "This is a spoiler and secret."
Would appear as: "This is a and secret."
Attachments are displayed automatically in the footer of the news feed item, but they are simply shown or not by checking the post's attach_count value and grabbing all attachments associated with that post.
Those attachment records have no awareness of the context in which they are used, so it would require some significant changes to exclude them.
We don't even do any actual BB code parsing here (we just strip anything that looks like a BB code) so it's not like we can even attempt to enumerate the specific attachment tags which are hidden by the spoiler tags as it stands.
I'll leave this open for a bit in case we have any better ideas but, sadly, this may end up being a design issue.