After some chat in HQ with Petr it seems that the role_unassign() function itself includes the necessary code to delete, for COURSE contexts, the associated user_lastaccess record if the user hasn't anymore the course:view capabilty.

But some sites can show orphan records that never are deleted from that funcion, so I would introduce one new iterator in cron.php in order to detect and delete all those orphan records from user_lastaccess.

Call this the ORPHAN_USER_LASTACCESS_PROBLEM from now. Patch attached. Requires approval, plz.

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Patch applied and working.