All users are able to access documents within projects in the MUX. There are users who only use the Classic view and users who only use the MUX. When the MUX users upload a document to a project the Classic view users are unable to see the document unless they are a participant on the Project. There is not a way to "Allow Non-participants to Access Document" when uploading from MUX.
User A is not a team member or participant of Project X
In Classic, a person who has the rights to upload a document can choose the option to "Allow Non-participants to Access Document." However, in Modern that option does not exist nor does it require any collaboration rights.
This feature does not exist in the MUX
Release : 15.8.1+
Component : PROJECT MANAGEMENT
This would be considered a new feature as it is working as designed.
Please review the below knowledge document on instructions for Enhancement Requests for Clarity PPM