Dynamic Attribute: Show no longer works

I had some output set to only display when the user is logged in (it’s actually a ‘Log Out’ link in the nav menu) which was working fine but now isn’t so I’m assuming it’s a bug introduced in the latest version.

The code generated is this:

<li class="nav-item" dmx-show="$_SESSION['siteSecurityId']">
  <a class="nav-link" href="logout.php">Log out</a> 
</li> 

I thought I was going mad and then wondered if it’s a new bug.