Why do you think the arguments are not passed to the task app? It works fine for me. I created a simple task that just prints the args:

dataflow:>app register --type task --name hello --uri docker://dturanski/hello-world-task:v2
dataflow:>task create --name hello --definition "hello"
dataflow:>task launch hello --arguments "app.mytesk.test=aaa"

The log file:

Starting: TaskExecution{executionId=5, parentExecutionId=null, exitCode=null, taskName='hello', startTime=Sat Oct 14 21:21:42 GMT 2017, endTime=null, exitMessage='null', externalExecutionId='null', errorMessage='null', arguments=[--spring.datasource.username=root,spring.cloud.task.name=hello, spring.datasource.url=jdbc:mysql://10.59.251.149:3306/dataflow, -spring.datasource.driverClassName=org.mariadb.jdbc.Driver, --spring.datasource.password=srz427UgQ0, app.mytesk.test=aaa, -spring.cloud.task.executionid=5]}
arg: --spring.datasource.username=root
arg: --spring.cloud.task.name=hello
arg: --spring.datasource.url=jdbc:mysql://10.59.251.149:3306/dataflow
arg: --spring.datasource.driverClassName=org.mariadb.jdbc.Driver
arg: --spring.datasource.password=srz427UgQ0
arg: app.mytesk.test=aaa
arg: --spring.cloud.task.executionid=5

Actually, the args are already shown in the standard Starting: TaskExecution message.