Which version of Joomla are you referring to?

Sorry, Joomla 3.9. In 3.8.13 it worked. The choosen element doesn't have the attribute -deselect but in 3.8.13 it has. Unfortunately, I wasn't able to determine in which part of the code the attribute is set (maybe in JS of the choosen library)

I confirm the issue.
This is what we got before 3.9.x

When clicking on the cross it deleted the position, thus setting it to None.
The cross is absent in 3.9.0 and staging.

@dgrammatiko

In chosen.query.js:
under 3.8.13, this.options.allow_single_deselect is true.
under 3.9.0, this.options.allow_single_deselect is undefined.

I don't know where this value is set.

I also try to figure that out and cannot find. So I guess, the chosen version has changed and that's the reason.

No change as we can see here:
https://github.com/joomla/joomla-cms/tree/staging/media/jui/js

Mysterious.
I can remember that we have had a similar issue back in a previous version. Unfortunately, I cannot remember which one ;-)

I found a way to solve this. can you test?
Modify
https://github.com/joomla/joomla-cms/blob/staging/administrator/components/com_modules/views/module/tmpl/edit_positions.php#L26

by using
'id' => 'jform_position_chzn', // i.e. adding _chzn to the id

The drawback is that we get in source a double _chzn for the <div id:

I can get rid of it in chosen.query.js by taking off + "_chzn" in

      if (this.form_field.id.length) {
        container_props.id = this.form_field.id.replace(/[^\w]/g, '_') + "_chzn";

But this may have undesired results.

BTW: I guess same issue when editing modules in frontend.

@infograf768 sorry but the error is not in the chosen file!!!
unset($this->_scripts["/media/system/js/moduleorder.js"]); and you'll see that the x button reappears. SO whatever was committed in the moduleorder script is wrong

Looking

I confirm.
reverting #21692 corrects this behavior.

@Fedik
Can you have a look at this please? I can't find how to modify the js except by reverting.

I will try

tks

Reverting is the best choice, that field doesn't need to be governed by chosen and if the styling (that was the issue solved there, right?) doesn't fit the rest of the chosen fields just update the CSS. DO NOT add javascript to fix CSS problems...

please test #23052

Set to "closed" on behalf of @Quy by The JTracker Application at issues.joomla.org/joomla-cms/22935