SPFarm MigratUserAccount does not migrate all groups

I have a SharePoint Foundation 2013 environment with FBA (about 6000 users). All FBA users are member of a "Central" SharePoint group which gives them access to the site.

I have build a solution which enables a FBA user to change his username. Therefore I use the SPFarm MigrateUserAccount command to migrate the users form his old username to his new username. The whole solution works fine, except the MigrateUserAccount command does not migrate the membership from the central group with more then 6000 members, all other SharePoint groups migrate succesfully. So the new member is not a member of the "Central"group which grants him or her access to the site.

In development where we have only a few members in the "Central" group MigraUserAccounts works just fine. Does anyone have similar experiences/difficulties with MigrateUserAccount and large SharePoint groups?