I don’t see any obvious problem with your logic.  Out of curiosity I coded a slightly different set of rules on my system and it worked fine:

WHENEVER Door (Zn 1) BECOMES NOT SECURE
THEN SET Msg Counter (Counter 1) TO 30

WHENEVER EVERY 1 SECONDS
AND Msg Counter (Counter 1) IS GREATER THAN 0
THEN SUBTRACT 1 FROM Msg Counter (Counter 1)

WHENEVER Msg Counter (Counter 1) CHANGES TO 0
AND DOOR (Zn 1) IS NOT SECURE
THEN ANNOUNCE Miscellaneous 10 (vm248)
THEN SET Msg Counter (Counter 1) TO 30

However, I’d personally utilize an unused Output to reduce the number of lines required to accomplish the same thing:

WHENEVER Door (Zn 1) BECOMES NOT SECURE
THEN TURN Output 37 ON FOR 30 SECONDS

WHENEVER Output 37 STATE IS TURNED OFF
AND Door (Zn 1) IS NOT SECURE
THEN ANNOUNCE Miscellaneous 10 (vm248)
THEN TURN Output 37 ON FOR 30 SECONDS