This one is sorted. I had been cutting power—but not removing the jack. I tried that, and the circuit power immediately fell. I used the same power supply with a different build. Same issue with the long LED fade after cutting power.
This is a wall wart I typically use just as a bench source and don’t typically use it with full builds. I powered the wall wart without a load and measured the potential. After cutting power, potential fell very slowly.
I’m thinking this is an issue with the power supply holding charge and discharging slowly through the low current load LED. Thanks all.