If the point acts like an entry/exit delay point when its prorammed as Perimeter Instant, the issue may be with the Part Mode Delay Time setting .
The Part Mode Delay Time setting is located in Panel Wide Parameters > Global Open Close Options > Entry Exit Timers. Enter 0 for an instant alarm.
If the Entry Exit Timer is set to a value other then 0, it will override the Entry Delay 1 and 2 for the particular points. Armed points that are not assigned an Entry Delay also follow the specified time, including instant points.