Something else might help you isolate this,

when I turn on "Data Capture Plus" feature in Datawedge (Zebra provided to setup buildin scanner), there is a “barcode" icon hover on screen to help capture barcode. If I use others app, like Zello, it works properly.  When I launch warehouse manager app and to the interface of picking/receiving, it will disappear. 

So, to me, that means warehouse manager bypassed TC20 default  scanner setting and directly control it. No matter how I change the setting. It won't works. Warehouse Manager App ignores ITF-14 barcode in purpose or never implement that decode.