DataMapper constant is missing in the output message

When I map a constant to some field, the field is empty.
Reproducible on 1.12.0-20210427 with AtlasMap M6.
E.g. transformation:

Output message:

Body: [[{"task":""}]] 

It works on 1.12.0-20210426 with AtlasMap M5.
Output message:

Body: [[{"task":"testTask"}]]