I am an experienced developer. As such tasks that are assigned to me get done usually ahead of schedule, which I always thought was a good thing. It so happens that sometimes there is some slack time between me finishing a sprint and the next sprint, and I use that time as I see fit - reading about new things, experimenting or simply browsing the news, and I didn't make any effort to hide it because I thought it would be unnecessary.
So I was called out explicitly about it and told that I should tell the manager that I was done and needed more things to do. I felt somewhat insulted, after all I did what was planned, it was approved and promoted to production without any problems. It gives me the impression that from their point of view I should pick up the slack of other developers, which doesn't feel fair.
What is the best approach in this type of situation? Should I just fill out the whole time allocated? Why not offer me the chance of learning something new and probably eventually beneficial for the company?