It has been observed that rules defined using the Modern UX functionality of Business Rules, do not work in Timeline Views, such as the Project Timeline view. Example rule would be to make the Name of a Project read-only. This rule works in the Properties page, in the Grid view etc. But does not work in the Timeline view.
Timeline Views have not be scoped under the purview of Business Rules.
An idea can be submitted directly to the Product Management team to introduce this functionality.
Areas of the Product that Business Rules apply to, are mentioned in the "Important" section of the following documentation.