I have an existing SSRS report which sometimes splits a group into two rows with part of the groups summary in one group row and the remainder in another. I can run the report several times (even different instances at the same time) and get different results; sometimes the group is in one row sometimes it is split into two rows. It is not double counting data, the data is splitting between the two groups.
I have redeployed the report and reset the subscriptions. Checked code to ensure the definition of the column for the group is consistent.