I didn't manage to fix the issue in core. I don't know why,

glpi/js/common.js

Line 1907 in daa142c

select2_el.bind('setValue', (e, value) => {

always receive the fist element of the array and not the passed array.

As I mentioned in the bug report, An array of data can be passed so it doesn't see the list of users as a single parameter.
https://api.jquery.com/trigger/

The trigger function takes an event as the first parameter. The second parameter is either a single value or an array of values to pass to the event handler. When the even handler is called, those extra values are passed as separate parameters.

You can see this yourself from the browser console:

$(window).on('myevent', (e, test1, test2, test3) => { console.log({test1, test2, test3}) } 

$(window).trigger('myevent', ["value1", "value2", "value3"]);

If you want to actually pass an array as a single value, it needs wrapped in another array:

$(window).trigger('myevent', [["value1", "value2", "value3"]]);

thanks @cconard96 but I finally used the ... operator to group all parameters in a single argument which is always an array (makes the received arguments consistent).