events.log does not include Item Updates. But based on the description, it sounds like the Item is being updated once per half hour. It is not unusual for bindings to update Items on a set polling period or periodically if the device reports it’s current state or the like. In fact, many users use this fact to find out when a device has stopped reporting using the Expire binding.
You can create a Rule that triggers when the Item receives an update to confirm (in OH 3 look at the events feed in the developer sidebar filtered on that one Item). If it is receiving the update, you’ll have to move the behavior to a Rule and a Timer.