I think you should still work with support, but in the past there was a few situations where clear messages were not being sent out. I believe most of these have been fixed over time though. You may want to track how exactly the alerts are getting cleared to look for patterns. For example, if you "clear" an alert by deleting the device itself, perhaps that might not send a clear message. Or if the clear occurs during a SDT or by the instance being deleted. Might explain why sometimes it works and sometimes it doesn't. In the past I've never really relied on LM always sending a clear but it's gotten better at it.
Also, I wouldn't suggest auto-closing tickets when an alert clears. That can lead to flapping alerts that noone looks at or situations like alerts that occur every night but clear by morning and it doesn't get looked at. I generally suggest to just add a Note to the ticket that the alert cleared, but still have someone look at the alert.