LIVE NEWS : the SmartThings platform/cloud is broken since 4 days ago, see : the Recent Platform Update Broke Parse Method Response/Result thread.
This bug had been making me crazy since the last 4 days, when none of my Handler modifications worked, whatever I did.
Sadly, at that time, SmartThings has not even aknowledged the problem, and the SmartThings Status is, erroneously, “all green”.
So stop any test with any Handler / Device until that one is solved.
This may also explain any behavioral oddity you may discover with other Devices, since basically it broke all/most STANDARD handlers for all/most Devices 