You are migrating a paticular Migration Object and while creating new records mandatory checks for Extension Fields are ignored by the Migration activity
Reproducing the Issue
1. Go to Business Configuration work center
2. Select Implementation Projects view or Overview.
3. Search and Open the migration activity Migration of Leads
4. Execute Migration for the respective Migration File
If a mandatory is set via KUT, it applies only for UI and not for Migration
To make the Extension Field mandatory in Migration a PDI validation has to be written to reject the save.
KBA , LOD-CRM , Cloud for Customer , How To