If there is a triggered alert and a decision is made to unmanage the related node, today this results in a reset of the alert and the related reset actions are initiated. However unmanaging a node should dismiss the alert and not reset the alert. Resetting, in my environment for example, means that a notification goes out saying the issue no longer exists which in this case is not correct information. As well an entry goes into the log stating the same which could be misconstrued when reviewing the log if it was not realized that the related node was unmanaged during the time the alert was in a triggered state.
So my suggestion again would be to dismiss any active alerts related to the resource being placed in an unmanaged state.