I have been experiencing sudden battery and data drain on my iPhone X and when I looked at the consumption I see that homekit is hungry data app. Has anyone else experienced this behavior? I resetted stats two days ago and homekit used 1.8GB of data since then!
I had this issue too. It only started happening after I switched on Settings > Cellular > Wifi Calling (not Wifi Assist).
I noticed my data usage was high and was alarmed to see under Settings > Cellular > System Services that HomeKit had chewed through 2.9GB of cellular data in just 4 days!
It seemed odd that that would be the culprit, but since I hadn't fiddled with any other wifi/cellular settings recently, I switched Wifi Calling off -- and only that, because I wanted to isolate the problem. It's been 3 days now and HomeKit data usage hasn't budged.
For me, this has solved the problem -- bizarrely.
I'm on an iPhone 12 running iOS 14.4, connected to AT&T.