Wrong result: range scan on a multi-part index with DESC key parts misses the row equal to the lower bound

With an index (a DESC, c DESC, b), a query with range conditions on all three columns returns an empty set although one row satisfies all conditions. Replacing a >= 5 by a = 5, or using IGNORE INDEX, returns the row.

How to reproduce

CREATE TABLE t(a INT, b INT, c INT, KEY k(a DESC, c DESC, b));

INSERT INTO t VALUES (9, 1, 1), (5, 5, 2);

SELECT a FROM t IGNORE INDEX (k) WHERE c >= 2 AND b >= 2 AND a >= 5;

-- 5   correct

SELECT a FROM t WHERE c >= 2 AND b >= 2 AND a >= 5;

-- expected: 5

-- actual:   Empty set

SELECT a FROM t WHERE c >= 2 AND b >= 2 AND a = 5;

-- 5   correct

Notes

The two-part versions (a DESC, c DESC), (a DESC, c) and (a, c DESC) return the correct result; the third key part is needed.