[ubuntu] Why is the group of my file changing when I move it?

I am probably missing something very basic here so maybe someone can help:

I have a file that I want do move from one folder to another. The file is created by the user jd and the group download. I work as the user maximilian. maximilian has the primary group maximilian and also is in the groups download and media. So I use the following commands:

sudo chown maximilian:media file
chmod 660 file
mv file /new/folder

But if I use

sudo mv file /new/folder

Can someone explain me what is happening there? If I move a file owned by a user and group to a folder belonging to the same user and group should it not stay the same?

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