Discussion of the bug that occurred on 26 June 2019 is in the following thread:
Arm & Disarm tile missing (SHM disappeared from V3 app 26 June 2019, had been working fine)
Also, it’s important to note that the “smart home monitor“ feature in the V2 classic app and the “smart home monitor” feature in the new V3 app are completely different code and have different features, they just happen to have the same name. So, for example, an SHM automation which is set up in the V2 classic app cannot be dismissed in the V3 app, and vice versa.
So the fact that a bug occurs in one of the apps and not the other doesn’t tell you anything. It’s not the same code. 