In order to switch your existing content templates back to the default editor you can do this by clicking the classic editor button in the right hand corner of your template at the bottom. See Screenshot
To disable this you can go to Toolset -> Settings ->Editor to use for Content Templates then select classic editor.
I don't want to use the Classic Editor. The problem is, when I open the content template 'Inbouwdetail content template test', the page 'Inbouwdetails' is loaded.
See screenshot below. Note the different URLs in the address bar. The right browser window is supposed to have the content template.
The test content template that you made loads just fine.
Recreating the content template is. It's used on a page with a block view. It was that page that loaded everytime I wanted to edit the content template.
My issue is resolved now, although I'm curious as to what was causing this. I don't want to have to recreate it again.