PWA-SharePoint - Access Rights / Settings

I am absolutely struggling to wrap my mind around settings for User Access involving PWA and the individual SharePoint Project sites.

In our solution:

1. We are using 'Project Permission Mode'

2. When a new Project is created, we have a Flow that creates a corresponding Team and SharePoint, and establishes a User selected while setting up the Project as the 'Owner' of the Team/Group for the Project.

3. Once the Project is created, in the Teams app, the 'Owner' will add Members to the Team for a specific Project.

4. Each Team has tabs which link to various document libraries (drawings, specifications, contracts, etc) in the SharePoint site for the Project.

The Issue:

When the Owner adds an employee to a Project Team, the employee is able to see that Team in their Teams app...

However, some employees, when they click on a Tab that corresponds to a SharePoint document library, they get presented with "You need permission to access this site / Request Access".

My main struggle is that not every employee experiences this issue, and I cannot find any sort of consistency.

Any advice?

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Additional Information:

1. Not all users need any sort of access to PWA or the PWA Project Site. The users that do need access to PWA are assigned (at minimum) a Project Essential license.

2. All users, when added to a Team, need (at minimum) the ability to Read the SharePoint site for the Project, and the files stored there.

Much appreciated,

TRR