if statement as replacement variable

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I'm trying to show a welcome message to all users who are not registered or not logged in on forumhome, forumdisplay and showthread. I had originally placed code

<if condition="$show['guest']">something</if>

in each of the three templates, but for the sake of efficiency should I want to update the welcome message, I assigned that code to a replacement variable.

The if statement evaluated correctly before I moved the code to a replacement variable. But now, it always evaluates the if statement as true, showing the guest welcome message whether logged in or not.

Does the replacement variable system not work with if statements? Or is there something special I need to be doing to make it function correctly?