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I noticed that I can't have a field with the same name in two or more different forms. Otherwise rules get merged, that is both fields need to comply with the rules from field A as well as from field B.
A solution might be to simply use two different names which is okay for custom forms, but not if you want to alter existing ones. In my case I have a comment reply form ("comment_form") under each comment on my page which the user can show if he/she clicks on the reply button. The comment_body field of the form for new comments has a rule set which those fields in the reply forms should not have. But they have due to this bug (or feature?).