The velocity chart and the sprint report should show an issue' story points value either of when the sprint started or when the issue got added to the sprint. During a sprint, SP values changes are visualised in the burndown chart (as a type of scope change), but these changes should not have an impact on either the velocity chart and sprint report.

Currently this is not the case. The sprint report shows the correct values, but the Velocity chart uses the values as of sprint completion time.