I'd like to know how it' possible to consider new sub-tasks as an additional and unexpected workload after a sprint is started?
According the Scrum and the Lean principles, the team isn't supposed to work on anything outside the sprint simply to avoid a waste of time is a US isn't elected.
Hense, the team start to work on a US only when the sprint really start. they split up the US into sub-tasks that can be done by different member of the team.
Saying that, why this strange Jira's behavior regarding the sub-tasks ?
The only issue is that the burndown chart is useless !