Side button on iPhone 7 doesn't silent anymore but declines the call
Under iOS 10 and earlier, the side/power button was responsible for silencing a call, just as you'd experienced. But now, under iOS 11, the volume keys — maybe just the Volume Down button? — are responsible for silencing a call. The side/power button remains the way to decline a call.

If you've recently installed iOS 11 on your iPhone, that may explain the change in functionality you've observed.

So, the next time you get a call, try pressing the Volume Down button and see if that achieves the effect you want.