Hi @Simone Kaden,
The most likely reason is that the fields your customer entered are on the create screen of the issue, but not on the edit screen.
The best way to check this, is to navigate to project settings > screens. There, locate the screen scheme associated with the issue type being used for the issues you are seeing this problem with. There may be screens for creating, viewing and editing an issue. If I'm right, you should be able to fix this by adding the missing fields to your edit screen.
(disclaimer: Jira administrator permissions are needed to modify the screen configuration).
Hope this helps!
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