There is nothing wrong that you are doing - it looks good its a expected behavior , if you change the trigger condition of the alert it would prompt a message "You have changed the trigger conditions for this alert. This will clear out any triggered alerts for this alert definition. Do you wish to continue? "
What happens in here is -> say an alert was fired with a previous trigger condition that you had, the same would be stored as an active alert in solarwinds and it would not trigger another alert for the same trigger condition until it gets reset (which is based on your reset condition). But then when you change a trigger condition and click on YES on the prompt that appears, it would flush out all active alerts related to this alert and it would start a fresh (note: in such cases if your component is still down, it would trigger another alert for the same and this might result in duplication of alerts for just one time but then you will have to live with it)
Basically when you change anything on trigger condition Solarwinds would internally convert the same to a SQL query and store it on the DB, thats the reason you get that prompt, hope it helps