As noted in
MDEV-24653, the function btr_cur_pessimistic_delete() is neglecting the opportunity to shrink the index tree when only one sibling page would remain after btr_discard_page(). The MDEV-24653 test case creates a B-tree that consists of two pages: the root page pointing to only one child page, and a leaf page containing one hidden record.
And as noted in
MDEV-14637, when fixing this, we would have to be very careful in order to avoid introducing deadlocks.