This happens to me too. I also used to use an HD500 through the S/PDIF with no issues into my Focusrite Scarlett 2i4. But I get the same dropouts with my Helix. I even tried a new cable, but that was a waste of money.
Until or unless Line 6 fixes this I’ll work around it. It still sounds great with XLR cables. But I wish they’d fix it properly, because as it is S/PDIF is useless on this unit and that’s disappointing since I paid for that feature and want to use it.
Doug