I have noticed this as well. I find more consistant ICE shut off occurs if I can get close to 40mph before coasting.

Other interesting behaviors I have found for urging the ICE to shut off:

Sometimes I need to accelerate from a stop to ~15 or 20 miles per hour then maintain or slowly coast up to another light. If I briskly accelerate (guessing about a quarter to half throttle) and suddenly let off the accelerator, most times the ICE will shut off. If I normally accelerate, it keeps running until I stop again.

I hit slow traffic during freeway rush hour - often 15 to 30mph speeds. I find if I can draw down the SOC a bit (one bar is enough), then the ICE starts shutting off anytime I let off the accelerator, even at speeds between 20 and 30mph. I don't know why, just that it is very repeatable. Once I get get the SOC down to the "ICE shut off zone", I can accelerate with the ICE (to avoid excessively depleating the SOC) and coast/maintain slow speeds in EV mode at will.

Despite taking 60 to 90 minutes to go 25 miles during rush hour, I often gain mpg on my tank average if I can get the SOC into the zone. If I can't, I typically loose a little mpg in rush hour traffic.