Steps to reproduce the issue

I created a menu item of type CategoryList pointing to a category which has several articles.
When going to the frontend, I saw the URL associated to the articles in the category list have the pattern:

https:///index.php/testing-menu/12-stereo-madness

and when clicking on it, ther is a problem because the page is incorrectly rendered. After a quick look, it is clear that something went wrong and the page source showed the problem:

Some of the resources are incorrectly referenced (as URL):

In this example, templates is the Joomla temaplate folder which is directly at the root level of the joomla website. We see here the "testing-menu" which is put before templates

I have a "Main Menu" which contains a couple of menu items. One of them is the one having the alias "testing-menu" and it has the type Article >> CategoryList.
The articles in that category are showed as links and looking to those generated URLs I see this kind of pattern:

https:///index.php/testing-menu/12-stereo-madness

This would have been OK but the problem is the rendering of the page. I don't know why Joomla resolves incorrectly that URL by looking for the main protostar css into a folder called "testing-menu".

I saw that the default menu item is OK. I tried to change the default on a different one and the previous defult became incorrectly rendered also.

I found also this topic which tells:
I am assuming your blog section doesn't have a menu item which is usually the reason for the category and article ID to appear in the url.

https://forum.joomla.org/viewtopic.php?t=941656

I tested by creating a menu item of type Single Article. Looking back to the generated URL for the article in the CategoryList, it bocame

https://index.php/10-faire-un-portail-du-nether

and it was correctly sourced in css and other resources.

So at present I see the following:

1. If no SingleArticle menu item exists, then the category and the article id are part of the URL. This leads to an incorrect resrouces links generation. -> Seems to be a Joomla bug.
2. Adding a menu item of type SingleArticle, the alias created with the menu item is automatically used in the point 1. and it leads to a correct page output.

Expected result

Clicking on the articles URLs generated by CategoryList menutype leads to article correctly displayed.

Actual result

The layout is incorrect. The cause is the bad link genaerated for the protostar template css (see steps to reproduce).

System information (as much as possible)

Joomle 3.8.1 (and before the very last update).

Additional comments

There could be a workaround as described in the final part of the steps to reproduce.