The STHM problems are a known issue and there are two different things that can cause it: preconditions having been deleted from your automations, which happened to some people, or changes to the “virtual switch“ DTH, so that you should use the “simulated switch“ DTH instead.
Both of these changes were undocumented and happened at about the same time, which is really annoying. 
See the following thread:
Outage Affecting STHM and some automations (19 Jan 2021)
Also note that SHM is no longer supported, so I suspect you probably meant STHM instead…
FAQ: SHM and STHM are different
If not, you should move over to STHM as SHM is likely to be flaky. but the problem you describe sounds like the one that has been affecting STHM.