When updating to 4.21, Element previously added to DataTable is missing. This only occur when you move to 4.21. If it change from 4.19 to 4.20, DataTable will keep data. it can restore broken data by reimporting .csv/.json data exported before updating the version.
1. Create BlueprintStruct with TMap members in 4.20.
2. Create DataTable based on step1, and adding TMap value.
3. Close the editor and switching engin version 4.21.
4. Open the editor and see the DataTable.
Then partial data of TMap type of DataTable is missing. The DataTable's data should keep the value even if the engine version is updated.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-68051 in the post.