Finally got a chance to try this setup out on my i3770K system using an old 250W HDplex DCDC, this unit still showed 15.V in off state, but at the same time I hit the power button, the inline meter showed DC dropped to under 9V, PC shutdown and supply came back up to 15.3V. Still waiting on another barrel adapter to try, but I'd say this cable induction chip is setup incorrectly.
Edit: The new adapter came in and I plugged into my Baseus 100W type-C male-male cable, output read 9V before plugging into the HP Prodesk, then went to 15.3V again, adapter has "18.5V" molded right onto it. Will attempt to run cpu/GPU benchmarks but don't think that'll get it up high enough but won't be able to run the i3770K again til after I move.
Edit2: Tried the meter plus adapter setup with my IOGEAR PD unit , oddly that settles in sightly higher on ~15.6V, still no 19V however this PC even running Heaven plus Prime95 simultaneously only has ~15W draw.
Edit3 - Tried my workplace Thinkpad Carbon X1 which draws almost 2A but still get the 15.xV, also tried at owrk from the adaptor that came with the unit. Nice thing was the meter confirmed my Wavtek IO extender is toast 4V output.
Everything I've tried from 3 ACDC Type-C supplies seems to confirm the PD negotiation is taking place, the meter supports 5-30V up to 5A, GC2.0 and QC3.0.