Writing this answer, I will assume that whomever is reading it, will be using an AUTOSAR Dem implementation from a vendor. If that is the case, look for "root cause" inside the vendor's reference manual.

This behaviour occurs because when you have a combined event, inside the stored data you will have a "master" event ID, as all combined events reference one event. That means whatever ID is stored could potentially differ from the one that triggered the DTC storage.

As a result, the Dem cannot employ status data restoration upon startup for the combined event after a power reset (the status block does not get saved in this case). To circumvent this issue, you need to add a second "root cause" event ID, either in the snapshot or extended data. This variable is internal to the Dem, hopefully, so you don't need to add any higher-level logic.