Update.
I figured something out. The comment forms only disappear when the user clicks the "comments" link on the front page / when reading article. (see image)
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The comment forms display just fine if the user clicks on the article title.
The forms stay if there are comments and if they click on the comment link like above / first picture. They only disappear when there are no comments.
Does anyone have any idea about that? I hope I am explaining it throughly.