I had a weird problem with HomeKit. It seems to have happened rarely, but I knew that the symptom occurred when my wife would call me at midnight because she woke up.
I followed her to the difference between two very similar commands.
"Hey Siri, turn on the lights in the living room" would work. Smoothly.
"Hey Siri, turn on the light in the living room" would light up a light in the living room and a light in the bedroom.
As it turns out, I had a light in the bedroom called just "light".
After renaming the light from the bedroom to something else, everything works great. As the saying goes, happy HomeKit, happy wife.