I currently have this issue and its really annoying.
I have a HomePod, iPad and iPhone X. The Homepod is in our Master Bedroom (upstairs) and I regularly have issues setting cooking timers in our kitchen (downstairs). The microphones on the HomePod are so good that my HomePod will pickup the "Hey Siri, start a 15 minute timer" instead of my iPhone or iPad sitting a few feet away from me. The only solution is to whisper or to physically hold your phone when your requesting commands from Siri which as one might notice, can be annoying when you have oven mits, water, raw meet on your hands etc. From what I've read the proximity of your iOS devices will come in to play and your HomePod will take precedence in responding assuming your other iOS devices are laying flat and your HomePod can hear the command.
Just my real world 2 cents worth...