The alert for crossing the Volume indicator with the value can be triggered when the volume of the new candle has not yet crossed the value set in the alert.

This happens because the Volume value at the candle's open is almost zero. And if the volume on the previous candle was higher than the value from the condition in the alert, then when the candle opens, this level is crossed.

This is clearly visible if you select to display the indicator as a line:

The screenshots below show that the volume crosses the set value, and the condition for triggering the alert is considered fulfilled:

If you need to know when the volume will be above or below a certain level, it is recommended to use the “Greater than” condition for alerts.