Finally made it outside during an actual rain (during the day so I could see) to discover why our yard was getting way more waterlogged than it should be during rainstorms.
Turns out our neighbor’s flexible drainpipe coming from their downspout either has a hole in it or has filled with silt right where it goes underground. Which turns into a small river running under both fences, and then hits the concrete block on the back of our raised bed and then unfortunately runs back toward our house instead of away, around the raised bed and into our yard.
Ah, this makes more sense now!
I talked to her daughter this morning; hope we can get this problem sorted out pretty soon. They’re in the middle of renovating their basement now, and the backyard is filled up with stuff, so it’s not going to be easy to get to that buried pipe to fix it. I think I’ll encourage them to just lay a new piece of flexible pipe on the ground running back to the alley for now as a temporary measure until they can do something nicer or buried.
It’s probably not the best idea to actually bury normal flexible pipe like that — I used 4-inch PVC instead and just flexible pipe from the rain barrel/downspout to the ground when I did our backyard re-do.