The snippet Profile has to be called uncached ([[!Profile]]), because the result is different for different requests.
When a placeholder is set by an uncached snippet, use an uncached placeholder tag to display it → [[!+username]].
(Using a cached placeholder tag ([[+username]]) might work in some cases, because the MODX parser is somewhat “lenient” and will postpone replacing placeholders it can’t find. But it will always break if you’re using an output modifier → [[+username:default=`null`]].)
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