At the moment, when a mentor quits (and doesn't opt-out using the M2 module), is removed from the mentors list by someone, or is blocked (long term block), their mentees are not reassigned. They have to be claimed one by one, as they apear to be active.
To have a continuity in service, maybe activated mentees (as in mentees who made at least one edit) should be automatically reassigned. (Suggested here). If automation is not possible, we should provide a way for communities to reassign mentees en masse.