I have a rather bizarre issue that I've run into - about 300 users (out of 1200!) in our instance had a security role that was created to help with the migration into our Dynamics 365 instance. This role was misconfigured and gave those users more access than it should have. These users also had another role that we had assigned that gave them all the access they needed. We then migrated their accounts, contacts, and activities from CRM On-Premise to CRM Online. We had a user accidentally delete a record from the system, so during troubleshooting I found out the misconfigured role and immediately removed it from the system. Upon doing so the users that had this role can no longer see any of their accounts/contacts even if they are the Owner of those records using the default views (My Active Accounts) and (Active Accounts).
If I did an Advanced Find on the Account entity with NO filters - nothing shows up. If I set a filter for 'Owner equals Current User' then records show up. Yet, the default view that has this exact configuration does not show anything, so in this case 'My Active Accounts' should produce results but it doesn't. I checked the view to ensure that it was indeed set up with that specific filter and no other.
I tried reassigning all records of a certain user to the users Team (since every Business Unit creates a Team automatically, I assigned it to that team for the business unit they were in). At first it seemed to work, I was ecstatic so I tried this with another user.. but it did not work. I went to Advanced Settings -> Security -> Users, found the user, opened their record and selected 'Reassign Records' when doing this.
I checked out Sandbox instance out since it had a recent backport of Production and found the role and checked to see if it was assigned to any teams, it was not. It was just assigned to those users.
I just don't know what could have caused these users to not be able to see their records after removing the role from Production. It shouldn't have affected anything - all of those records are either owned by the user or their Business Unit's team.