Occasionally a heavy tap can 'flex' on some large stainless steel sinks.
This can be rectified by your tap being braced. It is not unusual for this situation to arise, if you have a particularly heavy tap.
This is not caused by the tap or the sink being defective in any way.
To brace the tap, you will require a piece of wood or chipboard, which can be screwed across the underside of the worktop cutout. The tap can be fixed through a hole in this.
Alternatively, you can utilise a tap brace.
With a tap brace, the tap is effectively secured to the underside of the strut and not to the underside of the sink. This reduces any tendency for the sink to flex when the tap is turned on or off.
Franke have recently produced a new patented tap brace. This fits directly to the worktop and the tap brace has a variable collar which fits all models of taps.