When migrating from a Next-gen Project to a Classic SCRUM Project, the report history is lost thus all related reports are blank (Burnup report, Velocity report).
- Have a Next-gen Project where 1 or more Sprint occurred and completed
- Migrate (bulk move) from Next-gen to Classic SCRUM as described in Migrate between Next-gen/Classic
Issues migrated along with reporting history in the concerned reports.
There is no workaround at this moment. A possible solution to this would be to have a built-in migration tool such as JSWCLOUD-17441 where this data can be carried.