Steps to replicate the issue (include links if applicable):

• Go to a sandbox
• Write a nice poem - don't forget to use <poem> tags!
• Try to substitute a template into the poem, perhaps one that might help with formatting of said poem, such as {{subst:em dash}}

What happens?:

MediaWiki doesn't substitute the template, ignores the template and treats it as plain text, like:

{{subst:em dash}}

What should have happened instead?:

The template should've been substituted.

Note that the template works fine if you don't substitute it.

Software version:

1.43