Can't pass a document via start process

Are you having a user input task in the process after start node? I can see initially the cancel is blank ( in process history) but in variables it has value. So continue checking in history after which node execution does document becomes blank. 

The document is passed correctly at start but gets cleared or overwritten later in the process. Check for any nodes or logic resetting the variable after the start event.
Still not able to figure it out. Please share the process flow snapshot and details

Check input variables from interfaces...

Are you using  hidden variables?

Hello,

Can you check have to check the "Allow the value for the variable to be provided when starting a process" in your process model for the variable
attaching a screenshot for your reference