This is almost exactly what happened here.

I migrated my 10.3.9 Server with 190 accounts to 10.4.2.
All went well, all users were fine. Everything was and still is managed via mobile user accounts.
But when I started with entering new clients on the 10.4.2 server, these new clients failed to access their assigned shares. They simply did not see them.
Strange because the access privileges applied were the same as for the “old” migrated clients.
OS9 clients with the “new” account who tried to access the shares had no problem at all seeing the shares.
The “blind” clients run 10.3.7 or 10.4.2 or 10.4.3…
I found out that I had to add the everyone is allowed to read privilege to the share to give access to the new clients.
I should add that I do not use ACL but the inherit privilege model as it always had been used on our shares.
So today i created a demo Share gave access to groups but not to everyone.
The result is the same as mentioned above. With this test I wanted to clarify if there was something wrong with the old shares, but behavior is the same with new shares.
What am I doing wrong? It can’t be, that 10.4.2 needs read access for everyone for shares to make them visible.
regards

Oliver