Attached is a screen shot of the workflow and the code39 barcode string. The variable is taken form the Data Field drop down list
I just tried this on my system and got the expected results. What settings did you use when adding the Job ID variable?
Also, can you post a picture of the workflow?
I have noticed that when you place a jobID variable into a barcode (code39) and then split the job that the jobID comes out as "1".
In this code i have <jobId><Number of Sheets in Job><Sheet Number>
I imagine this occurs because the jobID on a split job becomes "number-01"