no, could not reproduce on omega.

Have you tried it with 4 or 5 category levels?  The problem arises with only 5 levels of categories.

Sample editor with 5 levels of categories

I changed OMEGA to BENDER leaving the categories at 5 levels. The problem persists, indicating that it is probably not a theme problem but an OSCLASS 8.2.1 problem with more than 4 levels of categories.

Sample of the problem in the image.

As I said many times, could not reproduce and sending other images will not help at all.

Please tell me the check you did was with 4 or 5 category levels. It's important to me because if you've checked with 5 levels and it doesn't raise a representation of the problem then I should look to see if there's a problem with the website hosting

Tested 5 levels, omega, osclass 8.2.1, jquery 3

You said in one of the answers that you were able to reproduce the problem in the BETA theme. Then what was the development?
(I'm sorry to come back to the topic but I do it because the problem exists and I can't solve it. and of course it's not possible to keep a problematic site alive)

Can anyone help resolve this issue with Osclass 8.2.1 (OSCLASS 8.2.1, OMEGA theme, PHP 8.1, jQuery Version 3.x.x)? As I show in the video when we want to edit an ad with 5 levels of categories and change the category of the item, then Osclass has a problem.

See in the video the problem I have.
https://www.youtube.com/watch?v=a-GQLIWZIBo

Unfortunately this problem on the publish/edit category page remains unsolved and the only way to overcome it is the "Interactive Select Box" option at least until the new version of Osclass comes out, hoping for its solution.

Can you reproduct here?
https://dev7.abprofitrade.eu/en/item/new

[email protected] / demo123

It's ahead of 8.2.1 with many adjustments

In the demo you sent, the categories work correctly. (With 5 levels of categories)

What could be possible causes of it not working properly for me? What should I be looking at?

Best is to wait for Osclass update.
There was a lot of work done cleaning and optimizing code, as there is option to define custom level depth.

Quick tip - in core, frm, item form file, there is javascript for loop and number "4".
Search for that for and change it to 8.

Can you please provide the file path.

I followed the path oc-includes/osclass/frm/Item.form.class.php and changed 4 to 8 as below and the category levels work correctly:
$('body').on('change', '[name^="select_"]', function() {
          var depth = parseInt($(this).attr("depth"));
          for(var d=(depth+1); d<=8; d++) {
            $("#select_"+d).trigger('removed');
            $("#select_"+d).remove();
          }
Is it correct?