555 timer circuits have been notorious unstable. At one point, I did design a circuit for automotive use that would implement about a 0-4 second delay. The parts are hard to come by, but I might be able to build a few if coaxed. One of the biggest issues was reliable resetting, just as you have. I solved it, but it doesn't work for the 555. The triggering is mostly edges from differentiators.
My circuit would reset every time if a) the timeout passed or (b) The input V went below about 4 to 6V (c) Initial power up from being off for a long time.
It was basically a delay on make timer with a protected bipolar transistor output on the high side. Commercial power on reset circuits need basically 100-150 mS to reset. I easily was able to meet less than 10 mS. but it was difficult to measure.