Scrum Guide says:
Product Backlog items that can be Done by the Scrum Team within one Sprint are deemed ready for selection in a Sprint Planning event. They usually acquire this degree of transparency after refining activities. Product Backlog refinement is the act of breaking down and further defining Product Backlog items into smaller more precise items.
But not every task can be broken down into sub-tasks that can be completed during one Sprint. More over, it just doesn't make sense to decompose every large task - some taks are just very cohesive and trying to decompose them just creates artificial impediments for developers. Insisting on such a decomposition would be counterproductive and artificial.
It looks like Scrum doesn't address this issue in any way?
- upgrading a software library to a higher version (requires of lot of small code changes all over the code base, fixing issues, debugging, etc)
- introducing an architectiral change
- investigating a bug
- configuring some software