Actually, I think there may be a bug that has been fixed but not released. Do me a favor and look at the App Status page (gear icon). There is an Event Subscription for your trigger, for the variable. Please tell me what that says as to the Handler.
Yeah, that's the bug, allHandlerX is the wrong handler. This will be fixed in the next release. In the meantime, you could get around the bug by putting the condition WeatherTemperature >= 18 into a Conditional Trigger for that variable *changed*. That will get the correct handler, and perform the same test. Evidently the bug was introduced along with the introduction of Conditional Triggers.
Note also that if this is a concern, note that you have the ability to downgrade via the Diagnostic Tool. (Getting help with a bug report in an old build is also unusual, but unusual things happen sometimes. )
Sorry, bugs are inevitable, and unfortunately adding new features incurs this risk. We make it easy to roll-back to a previous build, and we make every possible effort to find and correct bugs. Beyond that, we're just humans that can and do make mistakes.
Oh, and you did uncover another bug with your post: the missing log entry for Triggered, also fixed in the next release.