The only documentation on the spoiler tag that I could find is in Perl Monks Approved HTML tags:

Note that <spoiler> is not an HTML tag. It is used for presenting "spoilers" to questions and will be rendered differently according to the users preferences, with the general idea being that a little more work is required to see the content than just viewing a node.

So basically 3 out of the 4 options (in your Display Settings) for the spoiler tag break your intended use. Claiming that the spoiler tag is broken because it does not behave the way you think it should is pure hubris.

With that said, as I indicated in We need a second readmore tag so that people will stop misusing the spolier tag! it seems like a new tag should be created for the behavior you desire.

Note: The help for the Display settings no longer lists the options for the spoiler tag.

Update: Misuse of this tag has been discussed previously, e.g. in Re: Does PerlMonks do general code reviews? (seek).