When merging, we give the ability to choose the active sustainer during the "select master record" phase. If one of the records has an active sustaining gift, the Active Sustainer field is added to the "preferred fields" list to manually choose for the winning record. If the Active Sustainer field is not chosen during the merge, it will not be carried over during the merge process. Typically we would expect the master record to always be the active sustainer record, but if it's not, you'll want to make sure to make sure it is brought over by selecting that field when merging.