Every category and subcategory attached to every post

In Joomla, an article can have only one category. So in WordPress, the same post/category relation is imported.
So it is a normal behavior.

I’m not sure if I was clear about the problem – what I’m saying is, every single post in WordPress has 20 categories..
Here’s a screenshot
I don’t think this is normal behavior…

Originally, in Joomla, these posts only have 1 category, not 20..

You are right, this is not a normal behavior. I didn’t understand your request the first time.
Could you write me through the contact form and send me your Joomla database?
I’ll try to reproduce the problem.
https://www.fredericgilles.net/contact/