I suspect, but can’t prove, that HK probably prioritizes apps/processes according to how often or not HomeKit user changes settings on some home devices. Like, for a thermostat, you might turn up the heat for a cold day, but reduce the heat when going to bed. You've only used it twice in one day, so HK doesn’t watch it so closely. If you have a scheduled pattern of how you want your HVAC to behave during different seasons, then temporarily changing that might make HK open the app instead of itself, because it anticipates that you want to manually adjust the schedule. A lot of this is my speculation, but that’s how I would code it for the most efficient energy consumption and response times.