If I understand you well,
In your case, user task is followed by service task where completing the user task and executing the service task is part of the same unit of work and because of that data provided in user task wouldn’t be committed until service task is finished and therefore data is not accessible from the service task.
To be able to commit data of user task before execution of service task then you can set camunda:asyncBefore of service task to true as below
<serviceTask id="service1" name="Service1" camunda:asyncBefore="true" camunda:class="my.custom.Delegate" />
This way you put a transaction boundary before the service task to allow data to be persisted before the execution of the service task.