That's an American Products fixture, Pentair bough them out many years ago, now sold under their name.
How old is the fixture? If you are absolutely sue the lens/fixture assembly is right and tight, water could be entering from where the cord enters the back of the fixture. It's epoxied in, but sometimes the epoxy cracks, allowing water inside. That's usually a 25+ year problem.
Did you lube the lens gasket?
I always have.
Except a while back, the gasket I bought (because the usual one was out of stock) specifically stated in the instructions to not apply any lube to the gasket. So I didn't.
Less than 2 days later, the fixture had 3 inches of water in it. Learned my lesson.
Been working pools since 1995. Lube the gasket.