This should also be fixed by:
https://github.com/discourse/discourse-spoiler-alert/pull/12
[spoiler]Hello <b>Discourse</b> peeps![/spoiler]
Hello Discourse peeps!
[spoiler]
Hello **Discourse** peeps!
[/spoiler]
Hello Discourse peeps!
[spoiler]
Hello [b]Discourse[/b] peeps!
[/spoiler]
Hello Discourse peeps!
[spoiler]
Hello <b>Discourse</b> peeps!
[/spoiler]
Hello Discourse peeps!