I did a test and could reproduce this issue in my SharePoint Oline.
It seems to be the default behavior for SharePoint Online since it happens in all document libraries on all sites. I have also tried using PowerShell to set this field as read-only, but it does not work for this field either.
I recommend you give suggestion in the feedback portal and help to make improvements. Many features of current products are designed and upgraded based on customers’ feedback.
Thanks for your understanding.
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