Nested content to Block Lists

[IOSven]

We're currently upgrading an Umbraco v10 project to v13 LTS.

Nested content has been upgraded to Umbraco Block Lists in combination with Umbraco.BlockGrid.
All content has been migrated using the uSync.Migrations tool.
However, we notice that content pages with Block List items containing the data type 'Umbraco.DropDown.Flexible' are not converting properly, resulting in errors in both the backoffice and frontend.

The property value with the data type 'Umbraco.DropDown.Flexible' should have been migrated from the value "simple" to ["simple"].
This migration has been successful for every property except for those within blocks.

Any idea how to fix this problem using uSync migrations?