If you have a multimeter on hand I suggest doing a continuity check between PIN number 20 on each end of the existing DisplayPort cable.

If there is NO connection between the two ends on pin 20 then the cable is not a risk, at least in terms of this 3.3 volt DisplayPort cable back feeding problem reported by pca.

Displayport pinout

From 2012;
New Theory – displayport cables are not in spec

The broader issue is that cheating on cable specifications seems to be a real thing across all sorts of cable types. Many cable specifcation efforts (standards bodies) apparently allow the manufacturers to self-certify their products. And almost nobody is checking up on them.

I bought a 1000 foot spool of CAT5e outdoor rated cable last year that just does not like running at 1 gigabit speeds. 100mb works just fine, but of course that was not what I wanted. Cable was fully installed (4 runs) before the problem was noticed.