Frustrating - and just not up to the standard expected of SQ. It shows a lack of lateral thinking and communication between the crew.
You'll also notice that on the B777 and A380, there will be a Chief Steward/ess (red) devoted to leading cabin service in F/R and another leading the J cabin. They are directly responsible for service within their cabin, so it's a key leadership role. The in-flight manager therefore takes on more of a managerial role overseeing the whole aircraft from tip to toe.
On the A350, there isn't a Chief Steward/ess leading the cabin: the in-flight manager is in charge of the cabin, supported by a more junior Leading Steward/ess. This means the in-flight manager actually has to work within the cabin, rather than taking that supervisory managerial role. To me, that makes a very significant difference to how service is led and executed.