When replacing screws in an exterior light fixture, it's important to make sure that:
1 - None of the screws are loose;
2 - None of the screws are too tight;
3 - All the screws are tightened to the same tension.
Overtightening can cause the gasket to leak in between the screws. If you compress the rubber too much at the screws, it bulges out the sides and still messes up the seal.
Undertightening can cause them to leak at the screws.
It's like Goldilocks; in-between is just right.
Also note, incandescent lights can get very hot if they're left on for a long time. If the plastic lens feels hot to the touch after the light has been on for a while, the plastic could be warping just enough to cause a leak. The plastic wouldn't warp much, and would probably return to its original shape after it cools down. This effect is most noticeable on boat trailers, when a hot taillight gets submerged and cools off quickly; even though the light fixture is supposedly waterproof, it would still come out of the water with water trapped inside. On a travel trailer, the effect is more subtle.
Engineering: Finding complex solutions to simple problems you didn't even know you had.