Materialized Views: Index MV on TTL'ed column produces orphanized view entry if another column keeps entry live

CREATE TABLE t (k int, a int, b int, PRIMARY KEY (k));
CREATE MATERIALIZED VIEW mv AS SELECT * FROM t WHERE k IS NOT NULL AND a IS NOT NULL PRIMARY KEY (a, k);
INSERT INTO t (k) VALUES (1);
UPDATE t USING TTL 5 SET a = 10 WHERE k = 1;
UPDATE t SET b = 100 WHERE k = 1;
SELECT * from t; SELECT * from mv;

 k | a  | b
---+----+-----
 1 | 10 | 100

(1 rows)

 a  | k | b
----+---+-----
 10 | 1 | 100

(1 rows)

-- 5 seconds later

SELECT * from t; SELECT * from mv;

 k | a    | b
---+------+-----
 1 | null | 100

(1 rows)

 a  | k | b
----+---+-----
 10 | 1 | 100

(1 rows)

-- that view entry's liveness-info is (probably) dead, but the entry is kept alive by b=100

DELETE b FROM t WHERE k=1;
SELECT * from t; SELECT * from mv;

 k | a    | b
---+------+------
 1 | null | null

(1 rows)

 a  | k | b
----+---+-----
 10 | 1 | 100

(1 rows)

DELETE FROM t WHERE k=1;
cqlsh:test> SELECT * from t; SELECT * from mv;

 k | a | b
---+---+---

(0 rows)

 a  | k | b
----+---+-----
 10 | 1 | 100

(1 rows)

-- deleting the base-entry doesn't help, because the view-key can not be constructed anymore (a=10 already expired)