Why does my Thread lamp drop off the network every couple of days?

I've run a couple of those little C6 boards on my Thread network and had almost this exact pattern with one of them, and in my case it was signal, not sleep. Weak signal doesn't usally kill the device outright, it just means missed check-ins untill the parent gives up on it. Once the link is dropped, the board doesn't always re-attach on its own, so it sits there unavailable until a power cycle forces a fresh join. The lamp still working on the wall switch fits that perfectly, since the strip is fed separately and only the ESP side is going dark.

Before I'd blame the config, I'd do what we used to do with flaky RFM69 nodes back in the day, move the whole thing a metre from the border router and let it sit there a week. If it stays connected there, you've got your answer. Those metal bases are basially a faraday cage, I had the same headache with a steel downlight housing and ended up taping the board's antenna outside the shell with a bit of double-sided foam. Also worth checking whether it drops right after switching the lamp on and off, cheap switch-mode supplies can throw enough muck around to reset a radio. Ask me how I learned that one.

I had somethig similar with a nightlight I built and it turned out to be the cheap USB plug I was powering it with, not the board. Swapped in a proper phone charger and the random dropouts stopped. aparently those dollar store plugs sag when the little board wakes up and does its thing, and it just loses its mind instead of recovering. Might be worth trying before you solder anything. Is there an easy way to see how strong the connection actually is from Home Assistant, or do you need some special tool for that?

Yeah the signal theory lines up with what I saw too. Mine was inside a metal window frame and it would drop off every few days until I cycled the power. What actually fixed it for me was plugging a spare smart plug with Thread routing nearby, so the lamp had a closer parent to talk to. Downtime went from weekly drops to basically nothing over a couple months. If a power cycle always brings it straight back, that's the pattern I'd expect from a weak link rather than anything in the config.