Response from Mendix Support (Sep '24):
We checked the OData v3 spec and can see that it forbids this scenario too:
https://www.odata.org/documentation/odata-version-3-0/common-schema-definition-language-csdl/ subsection 6 "Entity Type Constructs":
The release notes of Mendix 10.0.0 contain the following note:
We had the same issue and did find an easy workaround with changing capitals which had no effect for our consumers: for example if the original attribute is "STATUS" change it to "Status". May not look nice in the definition, may not work for all kind of consumers, but for us the error was gone and our consumers didn't need to do any changes.