Just to make sure we’re talking about the same thing…
SHM was the security feature in the classic app.
STHM is the security feature in the new V3 app.
Although these have some similar functionality, they are completely separate pieces of code. You cannot dismiss an SHM alert from the new app or an automation created in the new app. And you cannot dismiss an STHM alert from the classic app.
If by chance you have multiple members of your household and one is using the new app and one is using the classic app then that can create the problem you are seeing.
Here’s the community FAQ. (The topic title is a clickable link)
If instead, you wrote SHM because you’re used to that from the classic app but you meant to say STHM, then that’s a different issue. Can you post a screenshot of the automation which is not working?