I’m gald there is a reason for the weird behavior seen.
NO service is completely reliable. Even companies that have millions of dollars a minute in lost revenue like Amazon and Google occasionally suffer outages and down time. myopenhab.org is no different.
For a truely free service (i.e. it doesn’t even collect information on its users and sell adds a la Facebook and Google) myopenhab.org is incredibly reliable.
But if you have a mission critical system like an alarm system you should have more than one method of delivery alerts in the first place. Almost every one of the Actions under Bindings are ways to connect to third party services that are designed for delivering notifications to phones including Pushover, Telegram, NMA, Prowl, etc.
Finally, for the masses of people who will take steps to integrate additional alert systems and/or change which service they you, I’ll recommend taking the opportunity to implement Design Pattern: Separation of Behaviors. This can even let you use a different service or more than one depending on the severity of the alert or time of day or the like.