Hi Thiago,
Thanks for the update.
We are glad to know that the reported problem has been resolved at your end. By default custom summaries is not support the Serialization and Deserialization. So when you Serialize and Deserialize the SfDataGrid, you have to disable the Serialization and Deserialization option for that corresponding summaries.
Please let us know if you have any further queries on this. We are happy to help you.
Regards,
Farjana Parveen A