the sollution you marked as "Accepted" cannot be the right one. This makes no sense i think
i had the same problem and figured out that it's a problem with the "default".aspx form. When you create a custom-form(old experience) with SharePoint designer and set it as default, then he bind it to the list. If you now browse with the browser through the Modern experience everything works.
But if you open the Element via link he open the binded custom-form(old-experience).
That means you must set your List-Formular to the old default "DispForm.aspx" Formular. This default Fomular is also binded on the modern experience.
If you accidentally deleted the default form, you must restore it. In the following video you can see how this works (The video shows how to restore with a library, but also works with lists): https://www.youtube.com/watch?v=DySGHRZT2Ko