It looks like GFM does not respect the![] alert syntax when it's enclosed within a div. This is true for both the coder.com GFM renderer, and GitHub's (though I assume they're the same internally).

When the section is surrounded by a<div>:

When it's not:

Presumably same deal on Coder.com:

In our case, we really want the tabs, and the alert block is less important, so we'll downgrade it to a regular quote.

cc@aqandrew for visibility, in case you're aware of a workaround. Merging this for now to remove the broken line.