Modernize your SharePoint team sites by connecting them to new Office 365 groups

Thanks for the response Bruno Aleixo 

Everyone except external users being removed from the Members or Visitors is by design.

I don't get it when you say "it is by design". I mean what could be the reason for that? Why after an indeterminate number of days? If it is by design, why not restrict it from happening in the first place so that the users don't get confused?

To workaround this you can..

Is this workaround a supported scenario? Or is it just a hack which can break anytime? (I am talking O365-Group-connected site here, to be clear)

...create a separate sp group with the same permission role

This makes no sense to me. If the three groups "owners", "members", and "visitors" are there already provisioned for the intended use, why is creating a separate group required? This is a problem and extra work. We will need to keep a tab on (and document) which sites an extra group was created and why.

Even the UI makes it very clear, that we are adding members to "site only" (not the O365-Group), and when "read" permission is given they get added to the "visitors" group. See this screenshot:

UI makes it clear that we are adding members to share site only

Now, your response does not make it clear as to why it disappears after a few days? Is a cleanup timer job running somewhere? If it is shared for only the site, then what is the need to clean it up in the first place?

In short, I am not comfortable in creating another SP group just for this, when the appropriate group is already there.

And while we are at it, how can we get "contribute" permissions for members instead of "edit". This is another big gripe of mine with O365-Group-connected sites. I agree, the traditional sites too provision the "edit" permission on the OOTB "members" group, but the big difference is that we can change the permissions on that group without the fear of it getting reverted on its own. But here, with the O365-Group-connected sites, I am wary of changing anything lest a so-called cleanup-routine will run and undo all changes after an indeterminate number of days!