This became known this while working on T330071: Mentorship: ensure that all mentees are assigned to an active mentor.
When a mentor retires, we reassign all their mentees to a different mentor, including mentees who happen to be currently blocked (whether temporarily or indefinitely). This is likely not necessary, and it sometimes causes confusion.
Recommendation: Silently drop the mentor/mentee relationship (if the block is lifted and user visits Special:Homepage again, they'd get a new mentor automatically).
Acceptance Criteria:
Given a mentee account is blocked,
Then silently drop the mentor/mentee relationship
If the block is lifted and the user visits Special:Homepage again, they'll receive a new mentor automatically.

