sortby random doesn’t work

Hi,
The random sortby function doesn’t work.
I tried the to set it up in ultimate admin and also in the shortcode with [sortby=”rand”]
, the second one seems better because i could see some change in the source code with random in data-sorting :

 class="um um-directory um-44b1b"
     data-hash="44b1b" data-base-post="2"
	 data-must-search="0" data-searched="1"
	 data-view_type="list" data-page="1"
	 data-sorting="random">

But when i reload the page, it’s always the same user …
If anyone as an idea about that ??? Thank’s