Data Migration: Default attributes missing in attribute aets if it is not in default Magento 2 attribute set

Preconditions

1. Magento 1.9.4.4, PHP 7.2
2. Magento 2.3.5, PHP 7.2
3. Data Migration Tool 2.3.5

Steps to reproduce

1. Source store has attribute sets with manufacturer attribute
2. Magento 2 default attribute set has not the manufacturer attribute in it
3. After migration, all migrated attribute sets are missing the manufacturer attribute

Expected result

Attribute Sets have the same attributes as the source store

Actual result

So far i understand following happens:
The Migration Tool copies the default attribute set of Magento 2 and inserts the new attributes from the old store. So the new Attribute Sets have no assigned manufacturer attribute.
It does not reassign the manufacturer attribute to the new / default attribute sets, if the source store attribute sets had this attribute.
Additional, the attribute values of all products are there till the PostProcessing.
The PostProcessing Cleanup deletes the manufacturer values from every product, because the attribute is in no attribute set.

Additional notes

When i add manufacturer to the default attribute set in Magento 2 before data migration, everything works.

Start by tracing the Data Migration Tool's attribute-set migration entry point and its PostProcessing Cleanup stage described in the report. Compare how source attribute-set assignments are copied when the Magento 2 default set lacks an attribute. Done means migrated sets retain the source attributes and cleanup no longer removes their product values.