This is pretty much the only thing I ever say to Siri for home-automation purposes, and I say it exactly like this, word for word. Today, Siri decided to get creative and try something different.
@siracusa A while ago I had to rename all my smart lights to 'lamps' because if I told Siri to say turn on the 'bedside light' it'll try to turn on every light in my house. When I renamed it to 'bedside lamp' and called it that in my command, it worked correctly. Seemed like any with request containing 'light' or 'lights' it default to controlling every light.