Add user to group using GroupManager seems to break some cache

Hi there!
I’m using GroupManager to add Users to Groups, which seems to work well as the user is indeed listed in the group but when a group is tied to an Issue Security Level, the user can’t see the issue (neither in quicksearch, nor in the detailed search). Weirdest part for me: typing the key in the quicksearch and submitting DO redirect the user to the issue (which is now searchable)…
When an admin does affect the exact same group to the user from the admin interface, everything works well…
Is there anything I should combine with GroupManager.addUserToGroup() to sync some cache or anything similar?
Thanks