Water leaking inside outdoor electrical box. What should I do?
That’s a standard weatherproof (Bell) box, and yes, sometimes they do accumulate and hold water as you have found.
Code not only allows boxes to be drained.
Code requires boxes to be arranged to drain; according to 314.15, a 1/8” hole is allowed.
I have found conduit bodies and boxes at a low point (at the bottom of a hill or conveyor) to fill from the bottom, and others to fill from the top. It’s not important, just drill a hole in the bottom and you won’t have any more problems.
So turn off the power and drill a hole and forget about it.