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.
Example:
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.
Any supported Clarity release.
This feature does not exist in the MUX.
This is working as per the intended design.
Depending on the Classic configuration for that project, the user will need to be a Participant or Collaboration Manager to see those documents in Classic. The Collaboration Manager can add the user from the project under Team > Participants > Add.