I want to ask, can the studio keep the bottom plate of our photos? Is this an infringement?

According to China's copyright law, authors enjoy copyright,

In other words, the photographer who took your picture has the copyright.

Therefore, in the absence of agreement, the other party has the right to keep the negative, refuse to delete it, or even refuse to provide you with the negative.

However, if the other party's behavior infringes on your rights, such as using your photos in commercial advertisements without your consent, or maliciously uglifying your image, it may infringe on your right to portrait or reputation, and then you can claim your rights.