This process is quite simple with a bit of variable analysis then start another workflow with a call activity in which the variable mapping looks like this:
In fact sorry to have bothered you with that. I found the bug in one of the first box of the called sub process. Based on many many checks it could happen that we do a :
execution.removeVariable("manualIp");
It was a quite rare condition and expressing the problem here already helped me to find what happened. So thanks to anybody who took the time to read this and sorry again.