Currently, on task completion or form submission, the variables are returned as follows:

{
    "aVariable": "aStringValue",
    "anotherVariable": 42
}

In other rest endpoints we are returning the complete information (type,valueInfo,...) of the VariableMap:

{
      "aVariable" : {"value" : "aStringValue", "type": "String",
                    "valueInfo" : { "transient" : true } },
      "anotherVariable" : {"value" : 42, "type": "Integer"}
}

It is a quick fix, add VariableValueDto.fromTypedValue [1] to [2] and [3].
[1]: https://github.com/camunda/camunda-bpm-platform/blob/3a765b00cd2e37de2186423b0f6f68036cfcba13/engine-rest/engine-rest/src/main/java/org/camunda/bpm/engine/rest/dto/runtime/ProcessInstanceWithVariablesDto.java#L52-L58
[2]: https://github.com/camunda/camunda-bpm-platform/blob/3a765b00cd2e37de2186423b0f6f68036cfcba13/engine-rest/engine-rest/src/main/java/org/camunda/bpm/engine/rest/sub/task/impl/TaskResourceImpl.java#L102-L103
[3]: https://github.com/camunda/camunda-bpm-platform/blob/3a765b00cd2e37de2186423b0f6f68036cfcba13/engine-rest/engine-rest/src/main/java/org/camunda/bpm/engine/rest/sub/task/impl/TaskResourceImpl.java#L135-L136