[JRTJH]

The burner tube on Atwood and Suburban water heaters is not much more than a curved 3" pipe that directs the propane flame in a circular motion from the burner orifice to the exhaust port. If there is a "large flame" caused by a delayed ignition or a "blocked tube" caused by insects or debris, then you may wind up with a "fire in the hole". If, what you saw, was a flash, that may be perfectly normal. In some lighting conditions, if you're looking at the exhaust port, you may see the reflection of the flame appear to be a "flash". If, on the other hand, you saw a "sustained flame" emitted from the exhaust, that's not normal.

I'd suggest cleaning the burner tube, then try again.

Chances are there's nothing wrong that a little "housekeeping" can't fix.