It looks like your browser is caching the CSS / JS from the old version, you can clear your cache and reload. In chrome use ctrl + shift + r to reload.

I was facing the same problem, First I thought it was a Cache issue but it wasn't. I checked the page source and found a css file being applied to the page which wasn't even present on my drive.

In my case the file was: static/admin/css/nav_sidebar.css

So I created new file in static/admin/css named nav_sidebar.css, then copy pasted the code which I got from page source and added "display:none;" in the #nav-sidebar section(line:34) and [dir="rtl"] #nav-sidebar section(line:45 in my case).

You can find the Original code here by using Google Chrome's inspect tool. Where to find the code

I had the same issue after bumping django from v3.0 to v3.1. I run python manage.py collectstatic to update css which solved my issue

I think the other answers here are necessary, but I'll add one more: You might have to bust caches in your server itself. In our example, nginx was configured to provide really long caches to our static files, so we just had to reset it to make changes come through.