DIAGNOSIS No, that's not normal and it's a sign you're lighting it incorrectly. A flash or *whoosh* means unburned gas has pooled inside the cooking box before you sparked the igniter.
FIX You need to follow the proper lighting sequence every time. 1) Make sure the lid is OPEN. Gas should never be turned on with the lid closed. 2) Turn the burner control knob to START/HI. You'll hear the gas flowing and the igniter clicking. 3) The burner should light within 3-5 seconds. If it doesn't, turn the knob to OFF, wait a full minute for any gas to dissipate, and start over. The key is igniting the gas immediately as it flows, not letting it build up.
IF THAT DIDN'T WORK If you're following the steps and still getting a flash, your igniter might be weak (see the battery/spark issue above), causing a delay between gas flow and ignition. Replace the AAA battery in the igniter module first.
HEADS UP This is a serious safety point. Always keep your face and body back from the front of the grill when lighting. If you ever smell gas at any time other than during the brief lighting sequence, turn the knob to OFF, disconnect the tank, and investigate for leaks before using it again.